tag:blogger.com,1999:blog-15650372392065992392024-03-13T10:45:38.050-04:00Joel Avrunin's Effective Bits of KnowledgeA Blog by Joel Avrunin on Electronics Test and Measurement, Focused on Oscilloscopes but covering a bit of everything!Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.comBlogger34125tag:blogger.com,1999:blog-1565037239206599239.post-86771322700153264992018-07-19T23:30:00.001-04:002018-07-19T23:30:18.144-04:00Joel Avrunin is starting a new job, but will keep bloggingI've been debating what to do with this blog since I started a new position. After 12 years with Tektronix, I have become the Director of Sales for Science Instruments at a company called Picarro.<br />
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<span style="color: #666666; font-family: Arial, Helvetica, sans-serif; font-size: 13px;">Picarro, Inc. is the world's leading-provider of greenhouse gas and stable isotope instruments, which are used in a wide variety of scientific and industrial applications, including: atmospheric science, air quality, greenhouse gas measurements, gas leak detection, food safety, hydrology, ecology and more. The company's products are all developed and manufactured at Picarro's Santa Clara, CA headquarters and exported to countries worldwide. Picarro's products are based on dozens of patents related to cavity ring-down spectroscopy (CRDS) technology. Picarro’s solutions are unparalleled in their precision, ease of use, portability, and reliability.</span><br />
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My plan is to keep sharing cool stuff I learn, so stay tuned!</div>
Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com0tag:blogger.com,1999:blog-1565037239206599239.post-38157815763658017152018-07-19T23:28:00.001-04:002018-07-19T23:28:13.429-04:00Video Compression - Best explanation I've seenI wanted to share this cool video I found on video compression. Tom Scott explains why video gets blocky when they shoot off confetti. I've tried to explain this to people a lot, but I've never seen it shown so well.<br />
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<br />Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com0tag:blogger.com,1999:blog-1565037239206599239.post-19454529628602212852018-01-09T11:35:00.000-05:002018-01-26T14:41:42.540-05:00Test and Measurement MetaphorsTo prepare for an upcoming sales meeting, I was ask to explain the value proposition of the Tektronix AWG5208. The question was - why did it accomplish a task for our customer that no competing product could perform?<br />
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For those who don't know, an AWG, or Arbitrary Waveform Generator, is like a reverse-oscilloscope. It takes samples in memory and "plays" them in the real world. They are very popular for developing new types of signal processing, such as MIMO Radar. Anything you can dream can be played. Later, hardware engineers can create a device based on the signals you create. AWG's are defined by basic specifications like sample rate (highest frequency signal) and dynamic range (or how small of a signal you can create in the presence of a large signal).<br />
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Sample rate and high dynamic range are fairly easy to understand, but I was asked, "Why is having 8 channels in one box a value? Can't you just use 8 signal generators tied together?"<br />
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In fact, we often do just that. To generate 8 radar signals, a customer could use 8 signal generators:<br />
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Or you could use 4 two channel AWG's:<br />
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Apart from the logistical problem of 8 signal generators, what is wrong with this approach? In T&M, I often look for a simple metaphor. So I grabbed this one. Suppose I want to measure this piece of wood?<br />
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But I only have four 12 inch rulers.<br />
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I count 3 complete rulers (3x12=36 inches) plus another 8 inches on the last ruler make 44 inches in total.<br />
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But suppose I have the correct tool, a tape measure. That tape measure is like 4 rulers in one.<br />
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Now I can easily see that the wooden block is really 45.5", not 44". Why did using 4 rulers not give me the same result as a single tape measure?</div>
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First, there is offset error. Turns out each ruler doesn't start precisely at 0. By lining up the wood block with the end of the ruler, I was adding a small amount of error with each 12 inch measurement. I laid the rulers end to end, compounding the offset error and also creating a synchronization error because each 12 inch section was not the same. And because the amount of "extra ruler" beyond the digits is different on each ruler, I might get a DIFFERENT measurement if I repeat and line the rulers up in a different order.</div>
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Offset error, synchronization error, lack of repeatability - all of these problems plague multiple instruments when they are tied together, whether oscilloscopes or signal generators. You can try locking references, but you can get high frequency phase problems. You can also get synchronization errors when executing complex multi-step AWG sequences as one can trigger slightly ahead of the other.</div>
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Can I work really hard to make 4 rulers take the place of a tape measure? Of course! But what is the fastest, most accurate, and most repeatable way to measure the wood? With the right tool!</div>
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On a side note, 8 channel oscilloscopes are also pretty useful....</div>
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<br />Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com0tag:blogger.com,1999:blog-1565037239206599239.post-6628757712449155412018-01-08T12:30:00.000-05:002018-01-26T14:54:28.235-05:00Joel Avrunin's Advice for College Hire Job InterviewsIn my position managing the US AE team, I have the opportunity to interview engineers at various levels of experience, from new college hires to senior level engineers. Having conducted more interviews than I can count, there are certain pieces of advice I would like to give to engineers looking for their first job out of school. My list applies mostly to engineers going into sales, but of course, much of this applies to any job interview.<br />
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Main caveat here - I am not a career coach or counselor. I'm just an employer sharing what I find are best practices in a college interview.<br />
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<li><b>Make your resume relevant.</b></li>
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If you are a college hire, that resume should only be 1 page and should contain only things relevant to the job at hand. Internships, jobs, notable group projects, etc. If your resume goes to 2 pages, don't go to a small font or make the margins smaller. Delete delete delete! Your college admissions counselor cared that you were an Eagle Scout and in the National Honors Society, but frankly, it isn't very relevant to me unless you can work that into the story of why you are well suited for the job. If you can make being an Eagle Scout into a good story, then by all means include it! Also, if you have a college degree, I don't need to know where you went to high school. Every single line of your resume should somehow communicate to me why you'd be good for this job.</div>
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By the way, not every piece of experience must be directly relevant as I mentioned with the Eagle Scout. Work experience such as internships or even unrelated jobs that show your work ethic do count! One of my favorite interviewees worked at Rite-Aid in college. Everything on a resume is fair game, so I asked him about it. He explained all of the roles he played in the store, and kept me occupied as he described the various aspects of the jobs. He was a good story teller about his experience (a plus in sales), and I knew that the attention he put on a job as seemingly mundane as operating a cash register would mean he'd be effective in a sales engineering job.</div>
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<li><b>Ensure your resume is true.</b></li>
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Everything on your resume is fair game to ask about. I had one candidate write "RF Transmission Line Design" under "Classes". Most candidates do not list their classes, and frankly it is not great practice in general. If you do list classes, confine it to junior and senior level specialty engineering - don't list freshman calculus! But if you do list a class, be ready to talk about it. I asked this candidate to define impedance. He couldn't. I wrote out Z=sqrt(L/C) and asked him to explain it. He couldn't. I explained the whole equation, and then asked if I made a microstrip wider, how would that impact the impedance. Blank stare. Finally the candidate said, "I didn't think I had to know this for this job". I told him, "You wrote you took a class on Transmission Lines on your resume, and this is fairly basic transmission line question. If you couldn't discuss it, then it shouldn't be on your resume. What else can't I trust on this resume?"</div>
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My favorite new college hire question is about group projects. Tell me about it. I then ask further questions on how the project worked. Or how it could be improved if given more time or resources. Very quickly I realize the difference between the group leader, and someone who was on the team but didn't do much. In other words, if you can't discuss a line item multiple levels deep, then don't put it on there.</div>
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<li><b>Look up your interviewer on LinkedIn</b></li>
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Especially for sales jobs, you must realize that an interview is a sales call where you are selling yourself. Look up your interviewer. First, the interviewer gets an alert when you look at his or her profile. I am always impressed when a candidate looks me up prior to the interview. I have only had 3 new college hires do this, and I hired 2 of them. You can see your interviewers past experience, connections you may share, even personal interests. If you looked me up, you'd find my blog and see everything you needed to know to have a successful interview with me. Last week I even posted the answer to my favorite interview question!</div>
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<li><b>Research the company on their website and LinkedIn</b></li>
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Spend some time on the company's website learning what they do. Read some white papers, news articles, etc. I am always impressed when a candidate has already been on the website. People want to hire people who want to work for them. If you put effort into learning about our company, it will reflect favorably on you.</div>
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<li><b>Act like you want the job</b></li>
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Continuing on the last point, you are under zero obligation to actually take a job offer. Therefore, there is no reason to equivocate during the interview. During the interview, make it clear how much you want to work for my company. Tell me why you want to work here. If you decline my offer, you won't be "in trouble" for acting like you really wanted to work here. If you are a good candidate, I will likely do some of my own selling during the interview. But the interview is not the time to play hard to get. If I don't think you really want to work here, I likely won't be too excited about hiring you.</div>
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<li><b>Don't drone on and on - answer questions succinctly</b></li>
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I had one manager who, as a rule, did not interrupt people he was interviewing. I witnessed him do this, and watched a candidate give a great answer in 30 seconds, and spend the next 5 minutes undoing their great answer. Give the answer and then be quiet and let the interviewer talk. As an interviewer (especially in phone interviews), it is frustrating when I can't get in a word edgewise.</div>
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<li><b>Dress for success</b></li>
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Yes - even for telephone interviews. Dress nicely, wear shoes, and stand up. You'll sound so much more energetic over the phone. I "meet" with customers over the phone, and I always dress as if I am meeting them in person even when I am in my home office.</div>
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<li><b>Bring questions and ask them, even if you know the answer already</b></li>
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When I asked you, "What questions do you have for me?", that is actually an interview question. Better phrased, I could say, "What questions did you prepare to show you are interested in this job and have thought about working here seriously?" Saying, "I have no questions" is the wrong answer. Even if you asked your questions to other interviewers, ask them again. I always leave at least 10 minutes out of a 60 minute interview for questions. Use those 10 minutes wisely and have questions to ask. And no - don't ask me how much vacation you get. The questions should be about the company, the work, our future vision, etc. Or get personal - ask, "Why do you like working for Tektronix? What's kept you here so long?" People like talking about themselves. I recognize these questions as good sales questions, and I give credit to interviewers who use them. They are all fair game and show you have a genuine interest in the company.</div>
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<li><b>Close - ask if you are right for the job and if not, why not?</b></li>
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If an interview is a sales call where you are selling your services, then you should close the interviewer like a sales person. Ask the interviewer to tell what your strong points are and where you are weaker. Ask if you are a good fit. The interviewer may refuse to tell you, or may share a potential red flag that you can alleviate. "You seem like a great fit, but I am concerned you've never had any leadership positions." "Oh? We didn't discuss my time as college debate team captain. Let me tell you about that....." I always give candidates credit for closing me, and the process can allow you one more chance to tell your story.</div>
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<li><b>Send a thank you e-mail</b></li>
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This one should be obvious. Send a quick follow-up thank you e-mail. Send it immediately because the interview debriefs are often the same day. Include a few bullet points if you must about why you'd be the best for the role. But get it sent quickly.</div>
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Hope that helps. And if you are interviewing with me and find this blog, not only have you done your research well, but you'll be ready to nail the interview.</div>
Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com0tag:blogger.com,1999:blog-1565037239206599239.post-2281382583571870592018-01-01T20:56:00.000-05:002018-01-26T14:42:34.920-05:00The Why and not the What of Receiver TestIt is important for an engineer in technical sales to be able to explain not just what "what" of test and measurement technology, but the "why".<br />
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I use this question in almost all of my interviews now for sales engineers, and it helps me see how well an engineer can explain a concept at a high level. The conversation goes something like this...<br />
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<b>Joel Avrunin:</b> "Many Gen 1 and Gen 2 high speed serial standards only involve transmitter test (TX), requiring an oscilloscope. Receiver test (RX) is limited or non-existent. I am an engineering manager putting someone else's silicon into my design, and I am concerned about the cost of test. So why do Gen 3 standards such as USB3.1 require RX test?"<br />
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<b>Here is the wrong answer.</b><br />
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<b>Applicant</b>: "Because it is required by the compliance test."<br />
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<b>Joel Avrunin: </b>"But WHY does the compliance test require it?"<br />
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<b>Applicant: </b>"To make sure the eye diagram is open at the receiver."<br />
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<b>Joel Avrunin:</b> "So I'll use an oscilloscope like I've always done to make sure the eye is open!"<br />
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<b>Applicant: </b>"But you can't do that - they are closed eye standards"<br />
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<b>Joel Avrunin:</b> "Why are they closed eye standards?"<br />
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<b>Applicant:</b> "Because there is too much high frequency insertion loss in the dielectric of the cable, jitter from random noise sources, ISI, and you need to make sure your CTLE is......"<br />
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<b>Joel Avrunin:</b> "WHOAH! Hold on... why can't I just use a better transmitter, higher quality cable, better interconnects, and get an open eye?"<br />
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<b>Applicant:</b> "Ummmmmm.... because it's a closed eye standard and the compliance test requires receiver test."<br />
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A sales engineer who works for me has to be able to explain the <b>purpose </b>of the test. We are entering an era where more attention is paid to cost of test than ever. When budgeting a new project, the cost of buying $1M of new test equipment (or even $100k) looms large over project budgets. Astute managers see labs full of equipment that was "new" just 5 years ago - why isn't that good enough? Especially outside the silicon world, receiver tests with products such as the <a href="https://www.tek.com/bit-error-rate-tester/bertscope" target="_blank">Tektronix BERTScope</a> just were not performed.<br />
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Everything the applicant said above was correct. But nothing there explained <b>why </b>receiver test is required.<br />
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Here is the simple answer.<br />
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Cost of interconnect.<br />
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It drives everything. Moving a high frequency signal 1mm across a piece of silicon is (relatively) simple. Moving it from the silicon, through a package, into a PCB, through a via - complex but still (relatively) simple. But get it to leave the safe world of the PCB and into a flexible cable and you start talking real cost to keep the signal intact.<br />
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This is a high quality, precision machined 2.92mm connector. USB3.1 is a 10GB/s standard, and this is what test and measurement companies like Tektronix suggest you use to qualify your designs.<br />
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It has bandwidth to 40GHz+, must be screwed in and torqued with a precision calibrated torque wrench to exactly 8lb-inch (no more, no less). The cable assembly itself has many layers to transmit the signal with little loss - most of these can cost in excess of $1000 and must be kept with rubber caps on the end to protect the threads of the connector.<br />
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We need that because fast data rates require fast edges from 0 to 1 and 1 to 0. A fast edge is a combination of frequencies. Fast edges with little jitter or ISI yield clear 1's and 0's (on the left below). Remove the high frequency content, and the edge slows down. Slow the edge down enough, and the eye "closes" and the data link fails to transmit data (on the right below).<br />
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<a href="https://3.bp.blogspot.com/-RN360jb0hyQ/Wkg-Glp94sI/AAAAAAAABcI/gSg91aZWrSMmUk7CtpY7-aFgDTc33P95ACLcBGAs/s1600/Analyze%252BSerial%252BData%252BLink%252BAnalysis%252BVisualizer%252B%25E2%2580%2593%252BTools%252Bthat%252Bopen%252Bthe%252Beye.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="252" data-original-width="381" height="211" src="https://3.bp.blogspot.com/-RN360jb0hyQ/Wkg-Glp94sI/AAAAAAAABcI/gSg91aZWrSMmUk7CtpY7-aFgDTc33P95ACLcBGAs/s320/Analyze%252BSerial%252BData%252BLink%252BAnalysis%252BVisualizer%252B%25E2%2580%2593%252BTools%252Bthat%252Bopen%252Bthe%252Beye.jpg" width="320" /></a></div>
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<a href="https://2.bp.blogspot.com/-2tC6r8p7IcU/Wkg_7c96O3I/AAAAAAAABcc/5lr_K8YNeIAD7xhTWBEdUmlRcAxXF_5RACLcBGAs/s1600/seagate_stea2000400_1_1.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" data-original-height="399" data-original-width="626" height="126" src="https://2.bp.blogspot.com/-2tC6r8p7IcU/Wkg_7c96O3I/AAAAAAAABcc/5lr_K8YNeIAD7xhTWBEdUmlRcAxXF_5RACLcBGAs/s200/seagate_stea2000400_1_1.jpg" width="200" /></a>This is a USB3.1 portable hard drive. 2TB of storage for $50 (as of the end of 2017), and it comes with a cable. You will likely stick this in the pocket of your cargo pants along with dryer lint and half a Clif Bar you are saving for later. Yet when you plug it into your computer, you expect to get 10GB/s of data transfer.<br />
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How do we pay $50 for the hard drive instead of $1000?<br />
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As you see below, the USB connector and cable assembly are not "lab grade". You can see that instead of 1 shielded and controlled impedance cable with a machined precision connector that is torqued exactly, we have a bundle of wires (shielded, but nowhere near as controlled as the cable assembly above), with a connector that uses pins that pressure-fit onto a PCB in a shield that is stamped and wrapped around the connector. It is a commercial quality connector, not a lab quality connector.<br />
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So how do we keep pushing data rates faster but keep the cost of our devices down? Better silicon receivers - we keep the magic in the silicon! That portable hard drive doesn't come with $1000 cable and a torque wrench. It comes with a $5 cable that you plug in and the silicon inside handles all of the impairment.<br />
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Let's revisit the interview:<br />
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<b>Joel Avrunin</b>: "I am an engineering manager putting someone else's silicon into my design, and I am concerned about the cost of test. So why do Gen 3 standards such as USB3.1 or PCIe require RX test?"<br />
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<b>Applicant:</b> "We want to move data rates faster than we did in the past, but we need to keep the cost of our interconnects down. In some cases we have legacy backplanes or CEM connectors that can't be easily upgraded. In other cases, we need consumer grade cable assemblies. Receivers need to handle impaired signals, and a BERTScope Receiver test ensures that these faster standards can still be transmitted over lower quality interconnects".<br />
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What do you think? Is there a better answer to this interview question? Please let me know in the comments section below.Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com0tag:blogger.com,1999:blog-1565037239206599239.post-44535382929995513322017-12-14T13:55:00.001-05:002017-12-14T13:56:23.604-05:00Using a VNA to measure an HF Antenna TunerThis is a great intro the using a VNA to measure an HF Antenna Tuner. There are some good low cost VNA options such as the new <a href="https://www.tek.com/vna/ttr500" target="_blank">TTR from Tektronix</a>, and Alan has a nice demo of how to use one. And of course, discussing the intricacies of using a Smith Chart:<br />
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<br />Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com0tag:blogger.com,1999:blog-1565037239206599239.post-63833639248722774252013-02-24T08:51:00.000-05:002013-02-24T08:51:11.812-05:00Is Science the Sum of All Knowledge - Video<iframe allowfullscreen="" frameborder="0" height="360" src="http://www.youtube.com/embed/PJyGeJSKdwg?rel=0" width="480"></iframe>Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com2tag:blogger.com,1999:blog-1565037239206599239.post-8484748515156764172013-02-12T15:44:00.003-05:002013-02-12T15:44:22.050-05:00Not so High on High-Resolution<br />
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Like many people in this industry, I love technology. Working for Tektronix, I am excited when we are the first to introduce new technology, like the Tektronix MDO4000 Mixed-Domain Oscilloscope. But I am also interested to innovation from other test and measurement suppliers. </div>
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Last year, Teledyne LeCroy introduced the WaveRunner HRO, or High-Resolution Oscilloscope. They rebranded it this year as the HDO, or High-Definition Oscilloscope. The idea is to use a 12-bit digitizer instead of an 8-bit digitizer to acquire a waveform. With an 8-bit digitizer, there are 256 voltage levels that define the wave shape. In theory, 12-bits means that there are 4096 distinct voltage levels, a great improvement in resolution. More recently, Agilent introduced the DSO9000H, a competing product that also promises 12-bits of performance through oversampling and processing. Such an oscilloscope should allow you to see small signals in the presence of big ones, provide greater accuracy of DC gain, and less noise on a signal.</div>
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So why doesn’t Tektronix provide a 12-bit oscilloscope? Aren’t more bits always better? Let’s review.</div>
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For those who are new to the industry, higher bit oscilloscopes aren’t a new idea. Below are examples from Nicolet (12-bits) and LeCroy (10-bits), but the idea never really caught on.</div>
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Why not? To begin, let me start with a simplified example. Below is a photo that I took back in 2006 when I started with Tektronix and visited the factory in Oregon for the first time. It is a photo of the garden and fountain in front of the main building on campus. The source of the photo was a high resolution RAW image taken on my digital camera, downsized to fit on this blog. The resolution is high enough that you can clearly see the sign reads “Howard Vollum Plaza Dedicated 2005” (Howard Vollum was the founder of Tektronix).</div>
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Below, I took the same photo and reduced the resolution even further. There are now fewer pixels in it and less detail. You can no longer decipher the sign in the garden.</div>
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With a simple photo editing tool, I was able to take the low resolution picture and “enhance” it back up to full resolution. However, despite the image having full resolution, you can’t read the sign. The difference between the image below and the top image is the source. In this case, the source lacked the detail I needed to make out the sign, even with full resolution. So more resolution does not provide me more detail because I have now introduced sources of error.</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">More resolution doesn’t mean more information</strong></div>
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Now let’s take a look at how this applies to 12-bit oscilloscopes.</div>
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The assertion with a 12-bit digitizer is that the individual voltage steps are smaller, so the waveform will have greater fidelity. In the figure below, the staircase is the digitizing level, or quanta. Suppose the signal was 100mV peak to peak, filling the screen of the oscilloscope. In theory, an 8-bit oscilloscope could only display a signal feature as small as 390uV (100mV / 256), but a 12-bit digitizer could show one as small as 24uV (100mV/4096). But how does this theory work in the real world?</div>
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The first thing is to remember that while high-resolution digitizers can be effective at low frequencies, they have far fewer effective bits at full bandwidth. Effective number of bits (ENOB) is the true resolution of the A/D once imperfections are included, such as non-linearities, gain errors, distortion, and noise. Just as the image above is a high-resolution representation of a low-resolution source, the same is true of an oscilloscope that has a high-resolution digitizer but other sources of error. If there is noise on the signal, that extra resolution is just extra bits of noise, and the waveform like the text in the sign above will remain blurry.</div>
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Hence, the first thing I did when I saw the new datasheets was look for “effective bits,” a common measurement you can find on Tektronix technical references. Unfortunately, neither LeCroy nor Agilent publish an effective bit specification. The actual “effective bit” is the effective quantization size once errors are considered. With increased bandwidth comes increased noise, and hence lower effective bits. Without this number, it is hard to evaluate the 12-bit oscilloscopes on their merits.</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">Making RF Spectral Measurements</strong></div>
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Since I could not find any reference to effective bits, I did find on the LeCroy WaveRunner HDO datasheet a Signal-to-Noise Ratio (SNR) specification of 55dB. SNR is a similar figure of merit to effective bits. In this case, SNR indicates how small of a signal can be discerned in the presence of a large signal. It would stand to reason that if the quantization step was smaller, then a smaller signal could be seen riding on a large one. Curiously, this specification is nowhere to be found on the Agilent DSO9000H datasheet. Since more digitizing resolution should indicate the ability to see smaller details (such as the Howard Vollum sign above), a “high resolution” oscilloscope should have an improved Signal to Noise Ratio. The Tektronix MDO4104-6 uses a specially optimized 8-bit digitizer in its RF path, and achieves a typical 60dB spurious free dynamic range. In narrow spans, the actual signal to noise ratio (excluding spurs) can approach 100dB. To learn more about how Tektronix accomplishes this task, there is an excellent paper available here: <a href="http://www.tek.com/document/whitepaper/secrets-behind-mdo4000-series-spectrum-analyzer-dynamic-range" style="color: #336699; line-height: 1.3em; margin: 0px; padding: 0px; text-decoration: initial;">http://www.tek.com/document/whitepaper/secrets-behind-mdo4000-series-spectrum-analyzer-dynamic-range</a></div>
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So from an SNR perspective, the 8-bit Tektronix digitizer in the MDO RF path is outperforming the reported performance of the 12-bit LeCroy HDO, and the performance of the Agilent 9000H is not reported at all.</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">DC Gain Accuracy</strong></div>
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Looking at other specifications, one would hope that since both LeCroy and Agilent advertise that they have 16x the resolution of an 8-bit oscilloscope, that the measurements would be 16x better. Yet consider a specification that they do report, DC vertical gain accuracy. Shouldn’t an oscilloscope with 16x the resolution be 16x more accurate? Yet the 12-bit Agilent DSO9000H has a reported 2% gain accuracy, versus a 1% gain accuracy for the 8-bit Tektronix DPO7000 series. The LeCroy HDO does improve this number with a laudable gain accuracy of 0.5%, but it is not exactly 16x better.</div>
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However, even the gain accuracy specification doesn’t tell the entire story. Suppose you put in a small positively biased signal (no negative swing) and the oscilloscope set to 10mV/div. On the LeCroy oscilloscope, the screen will go from -40mV to +40mV since there are 8 divisions. You need to set a -40mV offset to use the full digitizer range. Using their datasheet, the claimed accuracy of this 40mV offset can be off by up to 8.5%. With the same full scale settings, the Tektronix DPO7000 would only be off by 5.9%. So even a 0.5% gain accuracy advantage can vanish once you use the offset knob to center a signal.</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">What about those screenshots?</strong></div>
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But doesn’t the marketing literature from these two companies show 8-bit vs 12-bit versions of the same waveform, with the 12-bit one showing less noise and more detail? They do, but often the screenshots are cropped to not show the timebase. The timebase is critical, because a 12-bit oscilloscope will outshine an 8-bit oscilloscope running in normal mode when noise is not an issue, namely on low frequency signals such as those found in power supplies.</div>
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So does that mean 12-bit oscilloscopes are the tool of choice for power testing? Lost in this discussion is that 8-bit oscilloscopes do have a method for viewing low frequency data with 12-bit resolution, and that is called “hi-res mode.” Below is the exact same impressive screenshot you are used to seeing from those 12-bit oscilloscopes, but it was made with an 8-bit oscilloscope (a Tektronix DPO7000) being used properly. In this case, the main image shows the top of the square wave is fuzzy due to being sample with an 8-bit oscilloscope in regular sample mode. The inset image is the same signal capture with an 8-bit oscilloscope in Hi-Res mode, displaying the small ripple that increased resolution can discover.</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">Hi-Res Mode on an 8-bit Oscilloscope</strong></div>
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In hi-res mode, the data is significantly oversampled, and then a boxcar average is performed in acquisition hardware to real-time average even with single-shot data. In layman’s terms, it means that the 8-bit oscilloscope can provide up to 12-bits of detail thanks to its high oversampling. This makes the high-frequency 8-bit oscilloscope extremely versatile. You can use the high sample rate for your high-speed signals such as USB 2.0, but the hi-res mode for your fine measurements requiring up to 12-bits.</div>
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From this, it’s not clear that 12 bit oscilloscopes offer notable real-world advantages. Moreover, you have to consider that an oscilloscope is more than just its digitizer. There is the front-end, track and hold, and critically, the probe technology. Consider the new ASIC Tektronix developed for its front end which enables 1GHz passive probes (read more here: <a href="http://www.tek.com/document/application-note/improve-measurement-accuracy-and-reduce-cost-tektronix-passive-probes" style="color: #336699; line-height: 1.3em; margin: 0px; padding: 0px; text-decoration: initial;">http://www.tek.com/document/application-note/improve-measurement-accuracy-and-reduce-cost-tektronix-passive-probes</a> ). This allows probing of voltages up to 300V with a simple passive probe. The same technology enables the TPP0502 500MHz probe with a mere 2x of attenuation. For more detail, see my blog post on probe attenuation here: <a class="ext" href="http://www.effectivebits.net/2011/09/probe-attenuation-overlooked.html" style="color: #336699; line-height: 1.3em; margin: 0px; padding: 0px; text-decoration: initial;" target="_blank">http://www.effectivebits.net/2011/09/probe-attenuation-overlooked.html</a><span class="ext" style="background-image: url(http://www.tek.com/sites/all/modules/extlink/extlink.png); background-position: 100% 50%; background-repeat: no-repeat no-repeat; line-height: 1.3em; margin: 0px; padding: 0px 12px 0px 0px;"></span>. If you knock the signal down by 10x before the front-end, you are adding noise to the signal and reducing your effective bits. While 12-bit ADC’s are off-the-shelf technology today, the ASIC to enable the dynamic range of a passive probe with such performance is unique to Tektronix.</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">Head-to-head – Noise Performance</strong></div>
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I recently had the chance to test a LeCroy HRO with its 10x passive probe versus a Tektronix DPO5000 using its 2x passive probe in hi-res mode. Both oscilloscopes were matched in bandwidth to 500MHz. The first thing I decided to test was noise performance, because one of the biggest sources of error which reduces effective bits is vertical noise. If a 12-bit oscilloscope is to have greater effective bits than an 8-bit oscilloscope, it would be expected that the vertical noise should be much better. In a head to head test, I found that the noise advantage of the LeCroy was marginal without the probe, less than 0.1% full scale noise improvement, despite having a 16x quantization advantage. When I put a 10x probe on the LeCroy and a 2x probe on the Tektronix, the noise of the LeCroy was far higher at some settings because it was amplifying a more attenuated signal. The Tektronix DPO5000 was able to discern smaller signal details with less noise because an oscilloscope is more than just a digitizer – it is the sum of everything in the signal path.</div>
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I have yet to test an Agilent DSO9000H, but their datasheet does indicate some noise specifications at a single volts per division setting of 100mV/div. At this setting, it has a mere 0.2% noise advantage over an equivalent 8-bit Tektronix DPO7000C, and near identical noise performance to the 8-bit Tektronix DPO70000C series.</div>
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The conclusion here is that with no input signal attached, the 12-bit oscilloscopes appear to have marginally better noise performance, but hardly enough to make a difference in real-world measurements. Add on a probe and that marginal noise advantage can disappear quickly</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">Probes make a huge difference</strong></div>
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Similarly, many want to measure small amounts of current, and hope that a 12-bit oscilloscope can measure the top and bottom better in a switching supply. For a regular 100MHz 30A current probe, the Tektronix TCP0030 has a minimum full scale setting of just 10mA, versus 160mA for the LeCroy CP031. Once again, the less sensitive the probe, the higher the noise of the total measurement.</div>
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For extremely small voltages, the Agilent 9000H has 8 vertical divisions on the screen, and a minimum sensitivity of 5mV/div in high-impedance. Anything below 5mV is just a software zoom and has no additional bits of resolution. Put on a 10x probe, and the most sensitive setting is 8 divisions * 5mV * 10x, or 400mV full screen. If every one of the 4096 bits were perfect, the minimum quantization level would 400mV/4096, or 97µV. The Tektronix DPO5000 has 10 vertical divisions and a minimum sensitivity of 1mV. Add in the 2x probe, and the most sensitive setting is 10 divisions * 1mV * 2x or 20mV full screen. Divide it by 256, and the minimum quantization level is 20mV/256 or 78µV.</div>
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In a word, even if you assume that the 12-bits are perfect and ENOB was not a concern, then an 8-bit oscilloscope with a more versatile front-end and more sensitive probes can see smaller signals with less noise than a 12-bit oscilloscope.</div>
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Often there are other surprises, such as finding that the LeCroy HDO series does not appear to have a mixed-signal option for bringing in digital inputs. If a SPI bus is used in a power system, there literally are not enough inputs to trigger on the SPI bus while monitoring voltage and current.</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">Conclusion</strong></div>
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The conclusion here is that 12-bit oscilloscopes make for cool marketing, but they haven’t yet shown themselves to solve any measurement challenges over well designed and properly used 8-bit oscilloscopes. The digitizer may be 12-bits, but the system still has its own limitations. It’s possible that ADC technology will improve and the market will see high-frequency 12-bit oscilloscopes with benefits in some areas. When that day happens, keep your eye out for datasheets that reflect improved SNR, higher effective bits at full bandwidth, significantly lower noise, and more accurate DC gain measurements.</div>
Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com4tag:blogger.com,1999:blog-1565037239206599239.post-37153093376233636312013-01-06T21:28:00.001-05:002013-02-02T22:58:05.700-05:00Graduation Address - Is Science the Sum of All Knowledge?<br />
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<i style="line-height: 200%;"><b> Transcript of Joel Avrunin's Address to the Undergraduate and Graduate Students at</b></i><br />
<i style="line-height: 200%;"><b>Towson University's 148th Commencement Exercise</b></i></div>
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<i><span style="color: blue;">January 6, 2013 Commencement - 10 AM - Towson Center Arena</span></i></div>
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<i><span style="color: #38761d;">Graduation from <a href="http://mba.ubalt.towson.edu/" target="_blank">University of Baltimore / Towson University</a> Joint MBA Program</span></i><span style="line-height: 200%;"> </span></div>
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<a href="http://2.bp.blogspot.com/-orwB9Q7A36s/UOwvpGAKQ-I/AAAAAAAAAms/-_cyVsdZyu4/s1600/Speaking-Closeup.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em; text-align: justify;"><img border="0" height="320" src="http://2.bp.blogspot.com/-orwB9Q7A36s/UOwvpGAKQ-I/AAAAAAAAAms/-_cyVsdZyu4/s320/Speaking-Closeup.jpg" width="224" /></a><span style="line-height: 200%;"><span style="line-height: 200%;"> Thank
You Lisa Jackson our GSA president for that introduction. Good morning
President Loeschke, distinguished guests, honored faculty, family and fellow
graduates. With my undergraduate degree
in engineering, I sought to answer the question, “Is science the sum of all
knowledge?” Society accepts that if you
learn the science behind a system, you are now an expert who can tackle any
problem. With that mindset, I started at
Towson to become an expert in business, specifically wanting to know how to
manage organizational change. If science
truly is the sum of all knowledge, then just as an expert in the science of engineering
can design, an expert in the science of business should be able to manage. I foresaw going to class and learning the
skills needed to not only motivate employees and monitor their productivity, but
also to be an expert in all aspects of the business. My course schedule certainly read that way –
finance, project management, marketing, and accounting. And yet it was in an economics course that I
read the prescient words of Austrian economist Friedrich Hayek who asserted
that the knowledge of the circumstances of time and place were more important
than all of the science we can learn. Hayek
teaches us that since a manager cannot be at every decision point, he must
empower those he employs to make decisions on their own. He teaches that the further removed a
decision maker is from the point of knowledge, the slower an organization will
be able to adapt to change. </span><b style="line-height: 200%;">But if the
key to management is to be hands-off, then why go to business school at all –
what is the role of a manager?</b></span></div>
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To explain, imagine leaving this arena and driving up York Road to I-695. Cars speed
around Baltimore at over 60 miles per hour, and generally do not collide. How is that possible without a single manager
directing traffic? Isn’t that
anarchy? We stay relatively safe because
there are known rules for how to drive, and cars signal when they change lanes
or stop. An effective manager runs his
company like the interstate. He
establishes a common vision, the rules for the road. He acts as a conduit for communication, the
turn signals of the cars. But the
decision on which way to turn the wheel when a car comes too close – that is
made by the driver, not the manager. The
Maryland Highway administration would not respond as quickly as the driver
behind the wheel. When we discuss technological
marvels like the iPhone, we can often slip into honoring autocratic CEOs like
Steve Jobs at Apple. Why can’t all CEO’s
be visionaries like him? But when we learn
about the difficulties of working for such a controlling manager, how many of
us actually wish our own bosses were like Steve Jobs? We laud Apple’s successes, but forget the
same organizational culture that gave us the iPhone also gave us the failed Apple
Lisa of the 1980’s and Apple Maps of today.
<b>Could an empowered employee have foreseen
failure where the all-knowing leaders were blind?</b><o:p></o:p></div>
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Even in our own
lives, we instinctively value working not for the difficult person who
micromanages us, but rather the leader who enables our own success and creates
a strong organizational culture to support it.
My economics professor calls his management blog, <a href="http://www.givingupcontrol.com/" target="_blank">“Giving Up Control”</a>,
and that is the greatest lesson of management, whether in corporate America or even
as a parent. I was recently promoted to
sales manager and had to hand my old sales territory to a new person. Capable though he is, he will do things
differently than me. Yet he will not be
successful unless I give him the latitude to run things in his own manner,
while giving him coaching and mentoring as required along the way. As I proceed into my new management role over
the next few months, I truly am “giving up control”.<o:p></o:p></div>
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<span style="line-height: 200%; text-indent: 0.5in;">Those of us who
are parents have experienced the same phenomenon. Each of our lives is a sequence of thousands
of daily decisions we make on our own. Yet
we know that effective parenting is not making those decisions for our
children, but creating a common vision and framework so they are successful in
their own lives.</span></div>
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In closing, what
I’ve learned most from my Towson experience is that true success in management
comes not from doing the job of everyone else, but rather recognizing that when
an organization must adapt to new information, the best person to react is the
one nearest the change. The work of a
manager is to create a common vision for his team, empower each individual to
act quickly in a changing market, and to foster common language and
communication for his team to propagate knowledge for the benefit of the entire
organization.<o:p></o:p></div>
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Thank you and congratulations graduates!</div>
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<br />Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com2tag:blogger.com,1999:blog-1565037239206599239.post-64444222803973771602012-11-06T22:11:00.001-05:002012-12-09T22:15:04.658-05:00Why Would an Engineer Work in Sales?<br />
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Recently I had a chance to revisit my path from the design bench to the world of sales. Our sales organization was in the process of adding some field applications engineers (FAE), and I found myself advising prospects about how life would be different if they were to become FAEs. While I am currently a regional sales manager, my first job off the bench was as an applications engineer for Tektronix. Since this is a choice many design engineers may consider at some point, I thought it would be good blog fodder. Before you trade in your soldering iron for a minivan (mine on the left, Tektronix FAE Alan Wolke’s on the right), it’s important to consider all aspects of the FAE role.</div>
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Applications engineer can mean different things depending on who you ask. There are applications engineers based in the factory, such as those in the Tektronix Technical Support Center, who support customers from a distance, via the telephone, Web, or even Twitter. There are also applications engineers who work in marketing, functioning as the interface from engineering to sales and sometimes to the customer as well. Then there are applications engineers who support the sales force directly, many who perform additional engineering work to support the sale. It is in this latter category that we find the FAE.</div>
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In most cases, FAE’s seek to take a commercial off the shelf (COTS) product and explain how it fits into a particular user application. They are in front of the customer on a daily basis, doing both pre-sale work to explain the functionality of a new piece of equipment and post-sale work to train once the equipment is acquired. For example, an oscilloscope can do many things, but the FAE is the person who explains how it can be used in any given particular application.</div>
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The FAE is rewarding in that the job is almost always on-site at the customer location. He or she is the technical expert in the sales organization and spends most of the time on the road. This job could be at a test and measurement company, an FPGA or semiconductor company, or working as a product specialist for a technical distributor. Nearly every technology company has a need for FAEs in one capacity or another.</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">A People Person</strong></div>
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The first question often is, “Did you miss designing things?” The answer, of course, is that I did miss the work on the bench. It’s exciting to bring up a design from scratch, test it, debug it, and have total ownership.</div>
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But life in sales for me has been far more fulfilling. It may sound cliché, but I enjoy working with people as much as I enjoy working with technology. As a teen, I taught the basics of electronics and PC assembly (and “popular games” which was a class, believe it or not) to children in a computer camp outside of Boston. During college and my career, I’ve had a knack for explaining things and have always enjoyed presenting new ideas in front of a room.</div>
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Given my natural ability to converse with people, I felt that something was missing from my career. I had learned how to be an engineer on the bench, but felt I could do something different with my skillset. As an FAE, my job was to help blur the line between people and technology, making the complex easily understood. I actually experienced the same joy I had as a teenager teaching computer camp when I sat down with engineers and taught new ways to make measurements.</div>
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While you can become an FAE right out of school, it helps to have real-life experience. There is nothing than can replace the hands-on experience one can get working in the industry in design or manufacturing. The best college programs are no replacement for real-world experience. Real practical engineering must be learned by doing. Even if you grew up as a maker or hobbyist, the corporate environment is quite different from your basement lab.</div>
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Real-world experience helps the FAE better understand problems and needs from the customer’s perspective. It not only provides technical acuity to the FAE, but also sensitivity to a design engineer’s concerns when embarking on a new measurement challenge. Often an FAE must help an engineer explain to his own management why a piece of test equipment is needed for a new design. Understanding corporate structure and the approval process is essential to connecting to your customer.</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">Techno Playground</strong></div>
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Another exciting part about being an FAE is the absolutely huge breadth of technologies you are able to see. As a design engineer, you can easily get pigeon-holed into a single technology. The technology exposure of an FAE runs the gamut. My customers have been involved in DDR, PCI-Express, SuperSpeed USB, FPGA design, radar, satellite communications, satellite design, power supply design, high-energy physics, physical chemistry, spectrum monitoring, radio communications, rocket payload assembly, optical communications, and other applications I have yet to discover. One day I could be learning about what goes into a custom computer design for high-frequency stock trading and the next it is radiated emission spectra analysis of an unknown chemical sample.</div>
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I once heard Tektronix FAE Alan Wolke comment that when you are a design engineer, you will work on a single project for a long time, whereas being an FAE means “lots of little victories.” The life of an FAE tends to be centered on short-term deadlines, often consisting of a demo in a few weeks that you must prepare to present or a short program you write to customize a measurement. It is rare to have a project or task that extends beyond a month in duration. For those who like to always be working on something new, it is a great job to have.</div>
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Here’s another perk: you get the latest fun toys before anybody else does. When the Tektronix MDO4104-6 <a href="http://www.tek.com/oscilloscope/mdo4000-mixed-domain-oscilloscope" style="color: #336699; line-height: 1.3em; margin: 0px; padding: 0px; text-decoration: none;">mixed-domain oscilloscope</a> was introduced, it was on my desk before any customer ever had one. When the new real-time spectrum analyzer real-time upgrade was unveiled, my demo unit was upgraded immediately. When you are a design engineer, you often see these fun new tools on industry webcasts, but have trouble convincing your boss to invest in something new. As an FAE, it’s your job to play with new technology so you can teach others about it. Where else can you be paid to play with the latest and greatest test equipment?</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">Hit the Road</strong></div>
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Now let’s look at the tradeoffs of moving from the bench to the field. One is travel. Most FAEs spend a lot of time traveling. The amount depends on the company and how territories are structured. Some FAEs, such as those in high-density areas like Silicon Valley, likely don’t travel much. Others spend considerable amount of time in airplanes and hotels. A video applications engineer I know covers the entire area from Maryland to Florida and is constantly on the road. Since an FAE can in some cases put over 30,000 miles a year on an automobile, companies supply them with a company car.</div>
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Some engineers fear the idea of constant travel, especially if they have a spouse and kids. As with any career decision, consult your spouse before embarking on such a change! Airline points and platinum upgrades in hotels are nice, but nothing beats being home with the family. It’s a major consideration that prevents many from taking the job. And it’s not just air travel. There are times I have packed up my minivan with instruments and cables, driven four hours for a one hour appointment, and then driven four hours back home. I love driving but it’s not for everyone.</div>
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Others are excited by the idea of travel and seeing new places. When I first started with Tektronix more than six years ago, I was at the factory in Oregon for three weeks of training. I took the opportunity over the weekends to see some cool Oregon sites, like Howard Hughes’s Spruce Goose at the Evergreen Air Museum in McMinnville, OR and Haystack Rock (famous from the Goonies) in Cannon Beach, OR.</div>
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Other cool sites I have seen through my travels include the Indianapolis Motor Speedway in Indianapolis, IN, the Wright-Patterson Air Museum in Dayton, OH, the LaBrea Tar Pits in Los Angeles, CA, the NASA Goddard Space Flight Center in Greenbelt, MD and the NASA Wallops Flight Facility in Wallops Island, VA. Supporting my customers has taken me to cool places like aboard a ship to investigate RF interference and even down into an underground coal mine.</div>
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I was once driving to Rochester, NY for a customer demonstration and saw a sign that said Hammondsport, NY. I got off at the exit, wondering if the hometown of Glenn Curtiss would happen to have an air museum (yes I know my technology history a little too well). Luck was on my side: I got there 30 minutes before closing and they let me see some cool things including an original Curtiss JN-4D “Jenny.”</div>
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I’ve even had a brush with celebrity…I once waited in line to get my luggage scanned at Reagan National Airport behind the Stanley Cup!</div>
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Apart from the fun parts of travel, it’s also important to realize that most companies no longer have field sales offices, so working as an FAE often involves working from home and requires above average time management skills (and certainly more than 40 hours a week of work). You will typically support several different sales people, so communication and scheduling are important skills to master. A disorganized person cannot last long as an FAE. There is no water-cooler to chat over, so you must make time to stay in touch with your coworkers over the phone to maintain the office camaraderie that is critical for success. Many companies have annual sales training events, and that week is typically the only time you see all of your coworkers in one place.</div>
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Another consideration is that you must always have sales on your mind. While you don’t have to maintain a sales funnel and forecast revenue, you work in that world and need to be comfortable with reporting and revenue monitoring requirements. Depending on the company, the FAE may have 10-20 percent of his or her salary variable, based on commission.</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">Is it for YOU?</strong></div>
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If you are technically astute, like working with people, enjoy broad technology exposure, have a knack for explaining technical topics, and don’t mind some travel, applications engineering might be a perfect fit for you. It is certainly not what most people have in mind while they are in engineering school. I found myself enjoying the business side of being an FAE so much that I pursued a business career with Tektronix, and am currently a regional sales manager. However, while my responsibilities have changed, I can say unequivocally that being an FAE was the most fun job I have had and I certainly am glad to have had my career with Tektronix.</div>
<br />Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com5tag:blogger.com,1999:blog-1565037239206599239.post-34135585151071602422012-10-01T09:24:00.000-04:002018-01-26T14:46:14.988-05:00Putting the Logic in Logic AnalyzersCrossed posted at: <a href="http://www.tek.com/blog-entry/putting-logic-logic-analyzers" target="_blank">Bandwidth Banter Blog</a><br />
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<a href="https://www.tek.com/sites/default/files/media/image/Tek-tla6404-360_02a_h.gif" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" data-original-height="383" data-original-width="600" height="204" src="https://www.tek.com/sites/default/files/media/image/Tek-tla6404-360_02a_h.gif" width="320" /></a>Tektronix recently introduced the <a href="http://www.tek.com/logic-analyzer/tla6400" style="color: #336699; line-height: 1.3em; margin: 0px; padding: 0px; text-decoration: none;">TLA6400</a>, a performance leap in the world of value-priced monolithic bench-top logic analyzers. Performance like this used to require more expensive <a href="http://www.tek.com/logic-analyzer/tla7000" style="color: #336699; line-height: 1.3em; margin: 0px; padding: 0px; text-decoration: none;">card-modular systems</a>, more suitable to ASIC designers than FPGA programmers or general purpose users. However, with faster parallel bus signals (such as new high-speed COTS ADC’s and DDR memory), many designers find themselves needing performance logic analyzer specifications at budget-friendly prices. While a high-end performance logic analyzer can cost over $100k, the TLA6400 starts at around $13k.</div>
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All of that being said, many engineers think $13k is still too high, noting that there are now mixed-signal oscilloscopes (MSO) that have 16 channels of digital inputs, such as the <a href="http://www.tek.com/oscilloscope/mso4000-dpo4000" style="color: #336699; line-height: 1.3em; margin: 0px; padding: 0px; text-decoration: initial;">Tektronix MSO4000</a> series. In fact, for just a few thousand dollars, even the <a href="http://www.tek.com/oscilloscope/mso2000-dpo2000" style="color: #336699; line-height: 1.3em; margin: 0px; padding: 0px; text-decoration: initial;">Tektronix MSO2000</a>series can provide 16 digital channels of input with sample rates up to 1 GS/s. Some people call these “oscilloscopes with built-in logic analyzers,” a description I often correct because it really isn’t accurate.</div>
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There are also small USB devices that cost less than $1,000 which call themselves logic analyzers, such as the LeCroy LogicStudio. Often these devices hook up to a PC and offer from 16 to sometimes over 50 input channels, along with a PC interface for control. Just do a Google search for “USB Logic Analyzer”. There are dozens of various options out there from many different manufacturers.<br />
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What makes a true logic analyzer different from these little USB devices or a mixed-signal oscilloscope? Also, why does a true logic analyzer normally have 4 clock rates listed? The Tektronix TLA6400 for instance specifies 667 MHz, 1333 Mb/s, 3.2 GHz, and 25 GHz as max speeds, but what do these mean?</div>
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We can start with the easy specifications, but it will get more involved as we dive deeper. On a simple level, a logic analyzer is just a 1-bit oscilloscope. Instead of digitizing the whole waveform, the logic analyzer uses a single user-defined threshold voltage as a comparator. The signal is either high or low, 1 or 0. If a signal doesn’t reach the threshold, it is recorded as a 0, and if a signal goes above the threshold, it is a 1.</div>
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Unlike an MSO or a USB based logic analyzer, a logic analyzer can have over 100 inputs. It also has deeper memory. Many USB based products have less than 100k of memory, whereas a logic analyzer can have over 50 MB of memory. Also, a true logic analyzer typically can acquire at a faster rate. Most USB-based logic analyzers or MSO products go to 1 GS/s, but a logic analyzer like the Tektronix TLA6400 can provide timing resolution to 3.2 GS/s, and high-speed timing to 25 GS/s.</div>
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So a true logic analyzer has more channels, deeper memory, and faster timing. But what makes it truly distinct? The <a href="http://www.tek.com/oscilloscope/dpo70000-dsa70000-mso70000" style="color: #336699; line-height: 1.3em; margin: 0px; padding: 0px; text-decoration: initial;">Tektronix MSO70000</a>series mixed-signal oscilloscope has 12.5 GS timing and 125 MB of memory, exceeding the timing and memory specifications of some logic analyzers like the Agilent 16800A, but it is still not a true logic analyzer.</div>
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The key to understanding the difference is in how the data is acquired. Most MSO or USB based products operate asynchronous to the signal under test. On a true logic analyzer, this mode is called timing, asynchronous, or internal mode. In this mode, a logic analyzer’s timebase operates like an oscilloscope. It takes a sample at the clock rate consistently, and requires that the signal be oversampled to get accurate timing, just like an oscilloscope. The sample is still a 1 or a 0, but it is taken with an internal clock that must be greater than two times faster than the signals being analyzed.</div>
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However, the real power of a logic analyzer is in its ability to clock the inputs synchronous to the data being measured, by using an explicitly measured external clock line on the device under test. This mode on a true logic analyzer is called state, synchronous, or external mode. While some MSO’s (like the Tektronix MSO4000 and MSO5000 series) can do a synchronous parallel bus decode, they cannot acquire synchronously. They must take asynchronous, oversampled data and then post-process it using a clock that was also acquired asynchronously.</div>
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The advantages of synchronous capture are many, but the most significant is that your logic analyzer will see a true picture of the digital state of your system as your system itself sees it. Listing views and even disassembly of bus transactions like DDR rely on the data in the logic analyzer being clocked from the system clock under test. You also don’t waste memory with oversampling because the data is only clocked with the logic analyzer. Logic analyzers also support compound clocks, where multiple clock lines (and qualifiers) can be used to define a valid clock signal.</div>
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Some more advanced logic analyzers also permit time stamps to be put on the data as it is collected similar to asynchronous mode, but the data itself is only collected and stored on the clock.</div>
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Logic analyzers also feature advanced trigger state machines. Where an oscilloscope, MSO, or USB-based logic analyzer can look for a single logic state and trigger (sometimes with a simple second stage event), a logic analyzer can follow an elaborate state machine. For instance, it can look for Address A to occur 5 times followed by Address B within a time period of 100 ms in order to trigger. Each state can contain if-then-else clauses, with loops, jumps, counters, and timers. In the case of the Tektronix TLA6400, there can be up to 16 states with 16 nested if-then-else conditions per state. All of these trigger recognizers run at the stated clock speed as will be explained below.</div>
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Logic analyzers also have the ability to perform a task called Transitional or Conditional Storage. In this mode, the logic analyzer doesn’t store data if the storage condition isn’t met. For instance, the analyzer can be set to only store if a data line has transitions, so it does not store data if nothing on a particular line has changed. This is useful for bursty data buses with significant amounts of dead-time. Alternatively, it can be set to store only when a particular data word (such as an address bus) is set. A logic analyzer in this mode can capture minutes or hours of seamless data of interest.</div>
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As for specifications, what do those numbers mean on a Tektronix TLA6400? The first number, 25 GHz indicates high-speed timing, called MagniVu by Tektronix. In the TLA6400, all data is sampled by the 25 GS/s sampler, and 128 kB of that data is held for MagniVu while the rest is decimated for deep memory timing. As a result, the high-speed timing granularity is 40ps. The decimation rate for deep memory is the next number, 3.2 GHz, or 312.5 ps between points. This particular specification is a half-channel specification (with half the channels turned off). With all of the channels turned on, the state clock is at 1.6 GHz. This number indicates the speed at which the deep memory on the logic analyzer can be timestamped.</div>
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So if a system clock runs at 1 GHz, can a TLA6400 acquire it? The answer is a qualified yes, bearing in mind that a key distinction of a logic analyzer is its trigger system and memory controllers. And these features don’t operate at a full 3.2 GHz. Hence, the next number, 667 MHz, is the speed at which the trigger state machine can process the clock in real-time. If the system clock runs faster, the trigger recognizers will not keep up.</div>
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The fourth number, 1333 Mb/s, is the speed of the trigger state machine on a double-data rate signal (DDR), when the clock is on the rising and falling edge. Why is this specified since it is just two times the maximum clock frequency? The answer is because on some older logic analyzers, the hardware to handle DDR signals is a lower base frequency, so the DDR rate is not always twice the maximum single edge clock rate. For instance, an Agilent 16800A can trigger on a 450 MHz state clock single edge, but only 500 Mb/s acquisitions for DDR.</div>
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Another powerful feature on a logic analyzer is the suite of tools for analyzing signal integrity issues. For instance, in asynchronous mode, a poor termination or reflection can cause two crossings of a threshold between sample points. This condition is known as a glitch, and some logic analyzers like the TLA6400 can not only trigger on it, but mark the spot in memory where it occurred. In synchronous mode, the logic analyzer can monitor, trigger, and mark locations that setup and hold timing was violated. The Tektronix TLA6400 even has the ability to route up to four signals out of an analog mux to an oscilloscope so that they can be digitized and viewed in the analog domain. The image below shows a glitch in regular timing display (marked only in red), MagniVu display, and analog representation from the analog mux.</div>
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A logic analyzer is a lot more than a multi-channel 1-bit oscilloscope. While the MSO or USB-based logic analyzer provide digital inputs, most lack the synchronous-state mode clock, complex trigger state machine, conditional storage, and tools for analyzing signal integrity. Hence, the stand-alone logic analyzer still provides a compelling and powerful toolset to complement your bench and speed time to answer.</div>
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Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com0tag:blogger.com,1999:blog-1565037239206599239.post-72253543561496982062012-09-04T00:09:00.000-04:002012-10-03T09:31:24.078-04:00Solid-State Drives Offer Easy Speed Boost for ScopesThis is my first cross-posted blog post. I recently started blogging at <a href="http://www.tek.com/blog" target="_blank">Tektronix Bandwidth Banter</a>, so I will cross post my writings here. Link to <a href="http://www.tek.com/blog-entry/solid-state-drives-offer-easy-speed-boost-scopes" target="_blank">Bandwidth Banter Post</a> (same as below).<br />
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<tr><td class="tr-caption" style="text-align: center;"><em style="color: #4d4d4d; font-size: 12px; line-height: 1.3em; margin: 0px; padding: 0px; text-align: start;">The DPO5000 oscilloscope gives easy access to the hard drive bay.</em>
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Tektronix recently added solid state hard drive option to several of its higher performance oscilloscopes known as Option SSD. Why would you want a solid state hard drive, and what value would it add to your lab?<br />
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Anyone who has purchased a performance oscilloscope that runs Windows, whether a Tektronix DPO5000 series that is easily portable or a high end DPO70000 series with bandwidths up to 33GHz, might have noticed that the hard drive is easily removable. You typically need to remove just two thumbscrews to get the hard drive out of a modern oscilloscope. On a DPO70000, the drive bay is located on the back of the oscilloscope, and on the DPO5000, it is located on the right hand side as shown below.</div>
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This feature was added because many users wanted an easy way to remove sensitive information from their machines, either when sending the instrument out for calibration or to meet mil/gov security requirements. Some users buy multiple drives with their instruments so they can share an instrument, but not share sensitive data and setups. Since calibration data and serial numbers are not stored on the hard drive, changing drives to change labs does not impact oscilloscope performance. In addition, removable hard drives allow easy replacement of a failed hard drive without breaking any calibration seals on the instrument.</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">SSD prices drop</strong></div>
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The cost of solid state flash-based hard drives has dropped <a class="ext" href="http://www.tomshardware.com/news/Solid_State_Drives-Prices-Trend-ssd-drop,16013.html" style="color: #336699; line-height: 1.3em; margin: 0px; padding: 0px; text-decoration: none;" target="_blank">by over 50 percent</a><span class="ext" style="background-image: url(http://www.tek.com/sites/all/modules/extlink/extlink.png); background-position: 100% 50%; background-repeat: no-repeat no-repeat; line-height: 1.3em; margin: 0px; padding: 0px 12px 0px 0px;"></span> over the past year, as shown in the chart below. Unlike traditional spindle drives, solid state drives are based on flash memory, similar to the removable storage on a digital camera. As a result of the price drop, many computer manufacturers are offering SSDs as an option on laptop PC’s. In a mobile world, SSDs are appealing because of their reduced power consumption, immunity to shock and performance. Not surprisingly, many of the same advantages carry over to the world of test and measurement.</div>
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To start, many pieces of test equipment are used in high vibration environments. With its over-the-top handle, many DPO5000 series oscilloscopes are plugged into a long power cord and moved around a busy lab. In large industrial labs I have seen some creative (and scary) rigs used to hold oscilloscopes up on top of machinery. I once saw one with a long power cord actually mounted to a spinning table, trying to measure an anomaly that only happened during motion! Other instruments may be used inside vehicles or vans for mobile testing. Repeated vibration and motion can lead to early failure for a normal spindle-drive, but a flash drive is immune to these problems.</div>
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<strong style="line-height: 1.3em; margin: 0px; padding: 0px;">Faster boot up, processing</strong></div>
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The other main advantage of a flash drive is speed. Since there is no head that must move and wait for rotating platters, the SSDs improve the speed of saving, recalling, and processing data. I recently upgraded my demo unit MSO72004C with a SSD to see how it performed. I was amazed by the results. Oscilloscope boot-up took 55 percent less time, a greater than 2x performance increase than the spindle drive. Normal waveform update rate was not affected as Tektronix uses special ASICs for the display, but the time to save to a file to disk and recall a file were both cut in half.</div>
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I also tested high-speed serial, most notably rendering an eye diagram from a large dataset using DPOJET. This is a particularly time-intensive task because the Tektronix DPOJET software creates a software PLL to recover the clock from the serial data. I found that eye diagrams rendered in half the time as well. Continuing into the RF realm, I tested SignalVu, an RF tool that not only downconverts RF to baseband I and Q, but can demodulate signals into symbol tables and constellation diagrams. Running SignalVu on a large dataset, I found the speed increase was similar to DPOJET, running about twice as fast as with the spindle drive.</div>
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If you are looking to get more done in less time, solid state drives are now offered as options on almost all oscilloscope models. And, since the drives are so easy to remove, it is quite possible that you can order an upgrade kit for your existing oscilloscopes, depending on the age of the instruments, to greatly improve your processing speed.</div>
Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com0tag:blogger.com,1999:blog-1565037239206599239.post-80305206554182929192012-07-02T09:43:00.000-04:002012-07-17T14:16:43.457-04:00Analyzing SpaceWire Bus - Creating the Clock with Oscilloscope XOR<div style="text-align: right;">
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<a href="http://upload.wikimedia.org/wikipedia/commons/thumb/a/aa/SMAP_generic.jpg/300px-SMAP_generic.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="200" src="http://upload.wikimedia.org/wikipedia/commons/thumb/a/aa/SMAP_generic.jpg/300px-SMAP_generic.jpg" width="180" /></a>SpaceWire is one of the most exciting new technologies in the space electronics industry. Previous designs used <a href="http://www.tek.com/datasheet/serial-triggering-and-analysis-application-modules" target="_blank">MIL-STD-1553</a>, but are limited in speed to 1MB/s. <a href="http://spacewire.esa.int/" target="_blank">SpaceWire (IEEE 1355.2)</a> uses low-voltage differential signaling (LVDS) to push speeds from 2MB/s to 400MB/s. For those who are in the commercial world, it may not sound incredibly fast, but it represents a huge improvement in the harsh environment of a spacecraft. Today you can buy <a href="http://www.star-dundee.com/products" target="_blank">SpaceWire bus analyzers</a> to see the detailed protocol, but as designs move from faster, many designers are discovering the need to look at the actual bits for signal integrity work. Fielded designs are still using speeds below 100MB/s, but in the future it will likely be pushed to its limit. In this first blog post, I will discuss how to generate the clock using the MATH system on your oscilloscope. In future posts, I will discuss simple bit level decode and more advanced jitter and timing measurements.</div>
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Here you see a typical SpaceWire signal. The yellow trace is CH1 and it shows the data line. CH2 is the strobe. While data is carried on CH1, the clock itself is generated by the XOR of CH1 and CH2. Whether I want to figure out the bits, create an eye, or just see the clock timing, I need to generate that XOR signal.</div>
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<a href="http://4.bp.blogspot.com/-b3ljJkT9Uas/T2a3W4XotaI/AAAAAAAAAg0/H-FLy_xZmqs/s1600/SpaceWire-00.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="142" src="http://4.bp.blogspot.com/-b3ljJkT9Uas/T2a3W4XotaI/AAAAAAAAAg0/H-FLy_xZmqs/s320/SpaceWire-00.png" width="320" /></a></div>
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All of my analysis will be done with a <a href="http://www.tek.com/oscilloscope/mso-dpo5000" target="_blank">Tektronix DPO5204</a>, a 2GHz real-time oscilloscope that runs Microsoft Windows 7. SpaceWire signals are differential, meaning the signal has a + and - side, so I am using a <a href="http://www.tek.com/datasheet/differential-probes-0" target="_blank">TDP3500 differential probe</a> to acquire the signals for best signal integrity.<br />
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The first step is to use a MATH trace to translate the analog signal into a digital one. In this case, I am using REF2 instead of CH2 (because I am analyzing saved data). I simply make an equation that Ref2 > VAR2. I could have put 0 instead of VAR2 because this is a differential signal probed with a differential probe. The reason I used VAR2 is that often designers will grab a single ended probe and look at just a single side of SpaceWire. By changing VAR2 to a higher voltage (like 1.2V), the designer doesn't need to alter the MATH equation.<br />
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<a href="http://1.bp.blogspot.com/-8F-zkw6bRDA/T2a3Wki2xFI/AAAAAAAAAgw/pwf8aJCz5A8/s1600/SpaceWire-02.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://1.bp.blogspot.com/-8F-zkw6bRDA/T2a3Wki2xFI/AAAAAAAAAgw/pwf8aJCz5A8/s1600/SpaceWire-02.png" /></a></div>
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Here you can see the result. MATH1 shows the digital version of CH1/REF1 and MATH2 shows the digital version of CH2/REF2. Note that the edges have ringing on them. This is because the channel itself is either 0 or 1. The oscilloscope interprets this quick transition in MATH as an improperly sampled edge since the display uses SinX/X interpolation, it adds some visual ringing to the edge.<br />
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<a href="http://4.bp.blogspot.com/-QOPjg2nnEK0/T2a3XDVlXfI/AAAAAAAAAhA/BGPhZxtIuYU/s1600/SpaceWire-01.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="300" src="http://4.bp.blogspot.com/-QOPjg2nnEK0/T2a3XDVlXfI/AAAAAAAAAhA/BGPhZxtIuYU/s400/SpaceWire-01.png" width="400" /></a></div>
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To clean this up, I click on Display on the top menu, and set interpolation to Linear.<br />
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<a href="http://4.bp.blogspot.com/-fAdoiuo2xjA/T2a3XctkhwI/AAAAAAAAAhQ/zh_q8KTxZ5c/s1600/SpaceWire-03.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="300" src="http://4.bp.blogspot.com/-fAdoiuo2xjA/T2a3XctkhwI/AAAAAAAAAhQ/zh_q8KTxZ5c/s400/SpaceWire-03.png" width="400" /></a></div>
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<a href="http://3.bp.blogspot.com/-stzes-1NI6k/T2a3XHkgZmI/AAAAAAAAAhE/cJao2Id6r2U/s1600/SpaceWire-04.png" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" src="http://3.bp.blogspot.com/-stzes-1NI6k/T2a3XHkgZmI/AAAAAAAAAhE/cJao2Id6r2U/s1600/SpaceWire-04.png" /></a>Now comes the magic - I can easily XOR these signals even though I don't have the XOR function. In MATH3, I simply add MATH1 and MATH2. If either one is high, the function adds to 1 and is true. If both are low, the function adds to 0 and is false. If both MATH1 and MATH2 are high, the function adds to 2. Since 2 does not equal 1, my function is once again false. I have implemented the XOR function with simple comparative logic.<br />
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Of course, now my display is very busy. I am using 3 MATH channels when all I really need is MATH3.<br />
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Here I have combined everything into a single MATH1 equation. Now you can also see why using VAR1 instead of a real value is beneficial. If I change between differential and single ended probes, I don't have to edit the entire equation.<br />
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Below is my result - a clock has been successfully generated from my data and strobe. Next blog post, we will discuss how to decode the Data line into 1 and 0 values for export into a text file.<br />
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<br />Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com2tag:blogger.com,1999:blog-1565037239206599239.post-20631447772583769402012-05-07T12:31:00.000-04:002012-07-17T14:17:00.011-04:00What's Wrong with my Function Generator? (hint: nothing)<div class="separator" style="clear: both; text-align: center;">
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You sit at your bench and in front of you is a a <a href="http://www.tek.com/products/signal-generator/" target="_blank">function generator</a> and a basic <a href="http://www.tek.com/products/oscilloscopes/" target="_blank">oscilloscope</a>. You connect the function generator to the oscilloscope with a BNC cable and proceed to create a simple signal to measure. Surprise, the amplitude measured on the oscilloscope does not match what you set on the function generator. The sine wave may read 1V peak-to-peak (Vpp) on the function generator, and yet on the oscilloscope, it says 20Vpp or 2Vpp. Now is when you ask,</div>
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"What is wrong with my function generator?"</div>
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Commonly the question comes from a student and he is using a low to medium performance oscilloscope with a bandwidth of 200MHz or less. Examples include the Tektronix <a href="http://www.tek.com/products/oscilloscopes/tds1000/edu/" target="_blank">TDS1000</a>, <a href="http://www.tek.com/products/oscilloscopes/tds2000/" target="_blank">TDS2000</a>, and <a href="http://www.tek.com/products/oscilloscopes/mso2000/" target="_blank">DPO/MSO2000</a>, the Agilent DSO1000 and DSOX2000, LeCroy WaveAce, or really any lower cost oscilloscope about 200MHz or below.<br />
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I checked with the Tektronix<a href="http://www.tek.com/forum/" target="_blank"> technical support center</a>, and they confirmed they get this question commonly around the start of the semester when students are taking engineering labs for the first time. However, I have seen this misunderstanding trip up even seasoned engineers in other circumstances. The common issue is that these oscilloscopes have high impedance inputs only, and do not have regular 50 ohm inputs. Behind the single BNC connector, oscilloscopes that are 500MHz and above typically have 2 input paths switched by a relay. There is a 50 ohm path for high frequency signals and active probes, and a high impedance path for passive probes. So a <a href="http://www.tek.com/products/oscilloscopes/dpo7000/" target="_blank">2.5GHz oscilloscope</a> will have a 2.5GHz 50 ohm path and a 500MHz high impedance path.<br />
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A lower bandwidth scope is intended to be used with a probe, so it is almost always high-impedance only. High Impedance is abbreviated 1M or High-Z on many oscilloscope datasheets. Examples of such scopes are listed below.</div>
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In this example, I have set a <a href="http://www.tek.com/products/signal-generator/afg3000/" target="_blank">Tektronix AFG3252</a> to output a 1MHz sinewave at 1Vpp. Remember, the AFG is a 50 ohm device and is expecting to output into a 50 ohm system.</div>
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Taking the <a href="http://www.tek.com/products/oscilloscopes/mso2000/" target="_blank">Tektronix DPO/MSO2000</a> series as an example, first I push Default Setup and then measure the peak-to-peak voltage. I am shocked to find it is not 1Vpp, but 20.0Vpp! First, note that this particular oscilloscope has a probe with switchable attenuation. As a result, the oscilloscope assumes you have a 10x probe on the front. It takes the input voltage and multiplies it by 10 in software before displaying a waveform. You need to go into the channel menu and set attenuation to 1x. Most oscilloscopes of this type cannot recognize the probe, so they often assume 10x be default.</div>
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At this point, I still have a problem as you can see below. Any oscilloscope that is high impedance only will measure 2Vpp instead of 1Vpp. The problem is that the function generator is assuming I have a 50 ohm termination. Since I am using a 1M termination (and the scope doesn't have a 50 ohm input), I will always get the 2x scaling error. Looking at the screenshot below, the Pk-Pk voltage now reads as 2.00V.</div>
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So what solutions do I have?</div>
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Here, I have gone into the AFG output menu and told it that the load is High Z. What happens? Interestingly enough, the AFG does not change its own output impedance, but what it does is calculate the right voltage assuming a high impedance load. When I go back to the AFG screen, it now says my signal is 500mV.</div>
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So I have to bring it back up to 1Vpp, and notice in the top right corner, it says Load High Z.</div>
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<a href="http://1.bp.blogspot.com/-qxzs_ZioUIE/TpOETsjZrwI/AAAAAAAAAcM/0dIEEGT-jfU/s1600/AFG-HighZ-AmpReduced.BMP" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://1.bp.blogspot.com/-qxzs_ZioUIE/TpOETsjZrwI/AAAAAAAAAcM/0dIEEGT-jfU/s1600/AFG-HighZ-AmpReduced.BMP" /></a></div>
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Now my scope display matches my function generator.</div>
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<a href="http://2.bp.blogspot.com/-iaAC4ald2WI/Tt2F7dr2FfI/AAAAAAAAAfg/fEb6usymOQw/s1600/FxnGen-03.PNG" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" dda="true" height="195" src="http://2.bp.blogspot.com/-iaAC4ald2WI/Tt2F7dr2FfI/AAAAAAAAAfg/fEb6usymOQw/s400/FxnGen-03.PNG" width="400" /></a></div>
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Stick this signal into 50 ohms, and it will read 500mVpp, but read it on any of the high-Z only scopes listed above, and it will correctly be scaled at 1Vpp. Bear in mind, the AFG output impedance did not change, it just adjusted its own output voltage to account for the impedance mismatch.</div>
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Also remember: higher bandwidth oscilloscopes that include a 50 ohm input will default to high-impedance when you push "default setup". So everything I said above will apply even on a 1GHz or 2GHz scope if they have not been set to 50 ohms.</div>
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<tr><td class="tr-caption" style="text-align: center;">Tektronix TCP202 Current Probe</td></tr>
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The other place I see this problem is when engineers try to use current probes on lower end oscilloscopes. In one lab, I saw a <a href="http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13413&lc=EN" target="_blank">Tektronix TCP202</a> hooked through a Tektronix <a href="http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13507&lc=EN" target="_blank">Probe Power Supply 1103</a> into a DPO2024. As expected, the scaling was off by 2. The reason is that the TCP202 is expecting to drive 50 ohms, and since the scope only had a 1M input, the scaling was off as well. The solution is to use a<a href="http://www.testpath.com/Items/BNC-Feed-Thru-Terminator-50-ohm-2W-1GHz-117-007.htm" target="_blank"> feed-thru terminator</a> on the front of the scope. The feed-thru terminator is essentially a 50 ohm to ground termination, so the scaling from the probe is correct when an oscilloscope with 1M only inputs is measuring it.</div>
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<tr><td class="tr-caption" style="text-align: center;">50 ohm feed-thru terminator</td></tr>
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Hopefully when somebody searches for function generator, oscilloscope, and scaling, this post comes up and they quickly realize that you cannot mix a 50 ohm function generator with a 1M input scope without making the proper adjustments.</div>
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</div>Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com4tag:blogger.com,1999:blog-1565037239206599239.post-38231264938282925122012-03-02T16:34:00.005-05:002012-03-19T11:08:13.944-04:00EEWeb Featured Engineer InterviewCheck it out - I'm the Featured Engineer on EEWeb.<br />
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Here is the link - <a href="http://www.eeweb.com/spotlight/interview-with-joel-avrunin" target="_blank">www.eeweb.com/spotlight/interview-with-joel-avrunin</a><br />
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<img border="0" height="240" src="http://s.eeweb.com/resized/images/remote/http_s.eeweb.com/featured/member/2012/02/15/image1-1323269446-1329333121_675_407.png" uda="true" width="400" /></div>Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com0tag:blogger.com,1999:blog-1565037239206599239.post-88275828799394074552011-11-28T19:25:00.000-05:002011-12-09T12:36:34.852-05:00How to Make an Oscilloscope into a Network Analyzer<div class="separator" style="clear: both; text-align: center;">
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<span style="font-family: Calibri;">Ever been searching for a network analyzer but couldn’t find one nearby? Ever wanted to measure the front-end response of your <a href="http://www.tek.com/products/oscilloscopes/" target="_blank">oscilloscope</a>, but didn’t know how? A network analyzer can do a lot of things, but one of the most common tasks is insertion loss. Believe it or not, there is an easy way to do insertion loss without a network analyzer, and all you need is….</span> <br />
<a name='more'></a><span style="font-family: Calibri;">an oscilloscope with a math system and a fast risetime pulse.</span> </div>
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<span style="font-family: Calibri;">The traditional way to measure the frequency response of an oscilloscope is to feed a pure sine wave from a signal generator into the oscilloscope channel. Since the generator likely not perfectly level (especially at the end of the cable with a scope load on it), a power splitter is placed right on the oscilloscope front end. Half of the signal is diverted to a power sensor. The signal generator is set to a frequency (like 1GHz) and the user records the peak-to-peak voltage on the oscilloscope and the true power on the meter. Then, the generator is moved to 1.1GHz and the process repeats. While you can do this test manually, an external PC using GPIB makes it a lot faster... It's kind of like building your own network analyzer.</span></div>
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<span style="font-family: Calibri;">It is slow but accurate, and results in plots like the one below from a <a href="http://www.tektronix.com/">Tektronix</a> TDS6154C. This bode plot from a published white paper was likely produced from a calibration station making a measurement similar to above.</span></div>
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<a href="http://1.bp.blogspot.com/-vc1j4jSajRM/TtwPDF82F2I/AAAAAAAAAdo/go4o4poUL2I/s1600/ScopeNA-00-b.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" dda="true" height="203" src="http://1.bp.blogspot.com/-vc1j4jSajRM/TtwPDF82F2I/AAAAAAAAAdo/go4o4poUL2I/s320/ScopeNA-00-b.jpg" width="320" /></a></div>
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<span style="font-family: Calibri;">So how do we do all of this right on the oscilloscope with a fast ris</span><span style="font-family: Calibri;">ing pulse?</span><br />
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<span style="font-family: Calibri;">To explain, an infinitely fast rising edge should contain all frequencies from DC to infinity. Anything that slows or rounds the edge, whether it’s the characteristic risetime of the oscilloscope or the insertion loss of a cable (often called S21) is due to lost frequencies. By taking the FFT of the derivative of the edge, we can see the entire bandwidth response of the system. To characterize an oscilloscope, you need a pulse significantly faster, hopefully at least 2x times faster than the risetime of the oscilloscope.</span></div>
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<span style="font-family: Calibri;">Now, bear in mind that the result you get won’t be the actual bode plot of the front-end response because that is measured with sine waves, and the response of a system to a pure sine is different than its response to an “infinite” edge. In fact, I once tried this technique to measure a MEMS comb filter, and found that there was insufficient energy at the right resonate frequency to get anything through. Oscilloscopes are not specified with impulse response but rather bandwidth, and the risetime is mathematically related (this is a contentious point amongst manufacturers, but the equation "bandwidth = 0.35 / risetime" gets you close). Hence, a 2.5GHz oscilloscope should have about a 140ps risetime. Now this rule depends on the roll-off of the scope amongst other things – it gets you in the ball park, but is not an exact number. Look at the footnote on an Agilent or Tektronix datasheet, and you will see that while bandwidth is a guaranteed measured specification, risetime is always "calculated from bandwidth". It is generally not measured directly.</span></div>
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<span style="font-family: Calibri;">So first, put the rising edge in CH1 with extra time on both sides to get the low frequency content and reflections (unless the pulse generator is very non-flat between pulses, in which case you don’t want to include the generator slope). In MATH1, do a derivative of the channel. On a Tektronix oscilloscope such as this <a href="http://www.tek.com/products/oscilloscopes/mso5000/" target="_blank">DPO5104</a>, it’s MATH1=DIFF(CH1).</span></div>
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<span style="font-family: Calibri;">In MATH2, do an FFT of the derivative, or MATH2=SpectralMag(MATH1). Since the default vertical scale on a Tektronix oscilloscope is 20dB/div, I adjusted this one to 1dB/div. Also note that since I am at 10GS, the FFT covers 5GHz (10/2), but the useful information is under 2GHz since this is just a 1GHz scope (there is signal above 1GHz, but it is below 3dB attenuated). In the follow display, the purple line is my front-end response.</span></div>
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<span style="font-family: Calibri;">In this example, I have 2 cables I want to compare to see which one is more lossy. I am using a 12.5GHz oscilloscope for this measurement at 50GS/s, a Tektronix <a href="http://www.tek.com/products/oscilloscopes/dpo70000_dsa70000/" target="_blank">DPO71254C</a>. The pulse originates from a Tektronix AWG7122C, which should have intrinsic risetime of 35ps. First, I want to know how good first cable performs, so I connect the AWG to the scope and test it out. Here is the derivative shown in orange:</span></div>
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<span style="font-family: Calibri;">And MATH2 is the bode plot of the first cable I am using to evaluate my generator. It’s not very good and hits 3dB around 3GHz (my vertical scale is 3dB/div).</span></div>
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<a href="http://2.bp.blogspot.com/-7NIJFpZplrw/TtwPEIfbkAI/AAAAAAAAAeQ/OtqCemqw1uI/s1600/ScopeNA-04.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" dda="true" height="300" src="http://2.bp.blogspot.com/-7NIJFpZplrw/TtwPEIfbkAI/AAAAAAAAAeQ/OtqCemqw1uI/s400/ScopeNA-04.png" width="400" /></a></div>
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<span style="font-family: Calibri;">My next step was to save MATH2 to REF1 and then connect my second cable in the path with my now measured cable. The purple trace in M2 now shows the total loss. The white trace is REF1, my original measurement. But using MATH3 to subtract the two, I have in MATH3 the difference plot showing how much worse my second cable performs.</span></div>
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<span style="font-family: Calibri;">I did another interesting experiment when I sat in front of a LeCroy WaveMaster 816Zi for the first time a few years ago. I didn’t have much equipment with me, and was fascinated by the 3 different DSP settings, labeled Pulse, Flatness, Eye. A Tektronix oscilloscope has DSP ON and DSP OFF. Most performance oscilloscopes do not have more than 1 DSP setting, so I wanted to know their effect on the bode plot of the scope. I didn’t have time to sweep the entire front end, so I used a Tektronix <a href="http://www.tek.com/products/oscilloscopes/dsa8300/" target="_blank">DSA8200</a> with <a href="http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&ci=13571&cs=psu&lc=EN" target="_blank">80e10</a> TDR head to pulse (7ps risetime) on an oscilloscope with a specified 28.5ps risetime. Bear in mind, this technique does not verify the flatness of the scope to specification, but does give a good idea of how the DSP setting change response.</span><br />
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<span style="font-family: Calibri;">Below is a 7ps pulse on a 28.5ps risetime scope. It is interesting to note that FLATNESS (in green) has a ton of pre-shoot (ringing before the edge), characteristic of a DSP filter that is boosting the risetime to improve high frequency response. Indeed, it reads 28ps, just like the specification says. PULSE (in yellow) has little preshoot and a lesser risetime, likely to reduce visual distortion on the pulse.</span></div>
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<span style="font-family: Calibri;">As you can see below, the flatness setting really does just that – it gives flatter response out to 16GHz, but the DSP filter implementation in boosting the higher frequency creates a lot of preshoot, something undesirable for a person who wants to look at just a pulse. The risetime spec of the scope is given in flatness mode, presumably because the pulse setting sacrifices high frequency response and risetime for faithful pulse replication.</span></div>
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<span style="font-family: Calibri;">It is interesting to note that there is a 1dB hump near 11GHz. I thought perhaps my setup must be faulty, but then I read a paper LeCroy published showing their 30GHz raw response. I’ll link to it here: </span><a href="http://cdn.lecroy.com/files/whitepapers/real-time_digitizing_system_for_56_gb_dp-qpsk.pdf" target="_blank"><span style="font-family: Calibri;">http://cdn.lecroy.com/files/whitepapers/real-time_digitizing_system_for_56_gb_dp-qpsk.pdf</span></a><span style="font-family: Calibri;"> See figure 4 on page 3.</span></div>
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<span style="font-family: Calibri;">According to LeCroy, the bottom 16GHz of their 30GHz scope should be the same as their 16GHz scope (they use an intriguing technology known as "DBI" or frequency doubling to get to 30GHz). Comparing the response of the WaveMaster 816Zi to the bottom 16GHz of the WaveMaster 830Zi, the same characteristic hump appears at 11GHz in both places. Of course, I can't see the waveform without DSP applied, so I presume that the DSP improves the response from DC to 8GHz where the LeCroy published white paper makes it look like there might be some signal loss in that area. It is possible that the 3 DSP modes are used to tradeoff various DSP effects as proper correction of that hump may depend on the application.</span></div>
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<span style="font-family: Calibri;">The conclusion is that with nothing more than a fast pulse, I can measure a cable, and even measure an oscilloscope well enough to get decent correlation with a white paper that likely involved some pretty involved setup to measure.</span><br />
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<span style="font-family: Calibri;">You don’t always need a 7ps pulser to make use of this new knowledge. An inexpensive function generator often has an edge fast enough to measure 100MHz or so in insertion loss. And best of all, you'll impress your friends when they wonder how you got your oscilloscope to work as a network analyzer!</span>Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com1tag:blogger.com,1999:blog-1565037239206599239.post-83938856316679480912011-09-05T22:14:00.001-04:002011-09-05T23:44:16.406-04:00Probe Attenuation - The Overlooked Specification<a href="http://4.bp.blogspot.com/-3YVR6HCMYIM/TmAy6TBS3DI/AAAAAAAAAb8/88h9shFtyG4/s1600/ipodcar.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="223" src="http://4.bp.blogspot.com/-3YVR6HCMYIM/TmAy6TBS3DI/AAAAAAAAAb8/88h9shFtyG4/s320/ipodcar.jpg" width="320" /></a>The other day I plugged my iPod into my car stereo. The music was soft so I cranked up the stereo volume. The music was now loud enough, but it sounded terrible and hollow. Suddenly I realized my iPod volume was too low and I was amplifying a very small signal. As I turned up my iPod to supply a large signal to my stereo (and simultaneously turned down my stereo volume), I could achieve the same net loudness while getting a fuller sound. How does this relate to measuring a signal with your oscilloscope?<br />
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<a name='more'></a>The oscilloscope industry is abuzz in chatter regarding who has the best digitizer. One company will tell you that their 8-bit oscilloscope has more effective bits than another company's 8-bit oscilloscope. Another company introduces a "12-bit oscilloscope" with no indication anywhere of true effective bit performance. If you've ever attended one of my power supply measurement classes, you would know that hi-res mode on an 8-bit oscilloscope can provide >11 effective bits of resolution, and that anybody who sells you a 12-bit oscilloscope without specifying effective bits over frequency is likely selling you an impressive label more than a useful tool.<br />
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So the harried engineer wants to know which scope most accurately measures his signal, and yes, while effective bits is a good metric, there is something even more important than the oscilloscope itself that many overlook.<br />
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Before the front-end amplifies your signal, before the track-and-hold slows it down, and before the digitizer samples and stores the signal, the signal must pass through a probe. And the most important probe number, especially for power work is......<br />
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Attenuation!<br />
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Why does attenuation matter so much?<br />
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Imagine you just powered on your real-time oscilloscope and you need to probe your circuit. You could just use the probe that came with your oscilloscope, a standard 500MHz 10x passive probe. But this measurement needs to be accurate and you want to be sure you have the proper probe selected.<br />
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If you look online, there are lots of <a href="http://www.tek.com/Measurement/programs/advisor/">tutorials to help you pick a probe</a>. There's even an "<a href="http://itunes.apple.com/us/app/tektronix-probe-finder/id387826949?mt=8">app for that</a>" on your iPhone and iPad. There are passive probes and active probes, high voltage differential probes and high speed differential probes. Many oscilloscope users will decide their need (single-ended or differential), their voltage, and their bandwidth, and then pick a probe.<br />
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Remember back to <a href="http://www.effectivebits.net/2011/03/oscilloscopes-exposed-volts-per.html">my post on the V/div knob</a>. Each click of that knob adjusts the oscilloscope's front-end gain. It is amplifying your signal to fill the digitizer, from the top of the screen to the bottom. Modern oscilloscopes are able to read the probe attenuation value from the probe itself through a probe sense resistor (that springy pin on the BNC end of the probe). When the screen reads out a certain V/div, it is actually calculating it from the attenuation.<br />
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In other words, attach a 10x probe to the oscilloscope. Most oscilloscopes come with some sort of 10x passive probe. The industry standard is a 500MHz passive probe, although in the following screenshots I used a Tektronix TPP1000 1GHz passive probe. Either way, the probe actually divides the signal down by a factor of 10 as it presents it to the oscilloscope. Then the oscilloscope must reamplify the signal. In the following screenshot, the oscilloscope reads 200mV/div. However, the oscilloscope is actually set to 20mV/div, but the 10x probe it reads means the screen is effectively at 200mV/div.<br />
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This is also why you cannot see low V/div settings with a probe attached. If your minimum setting is 10mV/div, then with a 10x probe, your minimum setting is 100mV/div. Some oscilloscopes have a vertical software zoom on lower V/div settings as some do, but that doesn't actually give you any more information.<br />
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In the following screenshot taken on a <a href="http://www.tek.com/products/oscilloscopes/mso4000/">Tektronix MSO4104B</a>, I want to measure the noise and ripple on the top of my pulse. By drawing a histogram box (in brown), I can measure peak-to-peak and rms deviation, seeing that the signal is 131mV peak-to-peak and 18mV RMS during the on-period. If I were to try to go to a small V/div setting, I could reach 10mV on this signal as the MSO4104B has a true 1mV/div setting on it. 10 x 1mV/div = 10mV/div. The 1mV/div setting is bandwidth limited, but it is not a zoom.<br />
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Also note that I made these measurements using "hi-res" mode and running at 500MS/s. <a href="http://www2.tek.com/cmswpt/faqdetails.lotr?ct=FAQ&cs=faq&ci=3855&lc=EN">This should provide me about 220MHz and 9 effective bits</a>.<br />
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<a href="http://3.bp.blogspot.com/-qLeeQ5ejDzg/TmAwK7bx8VI/AAAAAAAAAbw/0frOl6N8n38/s1600/TPP1000.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="480" src="http://3.bp.blogspot.com/-qLeeQ5ejDzg/TmAwK7bx8VI/AAAAAAAAAbw/0frOl6N8n38/s640/TPP1000.png" width="640" /></a><br />
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To get a better measurement, I tried a different probe. This one is also a passive probe and is 500MHz, but is only 2x attenuation, the <a href="http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=17795&lc=EN">Tektronix TPP0502</a>. Remember the problems I had when I tried to amplify the quiet signal from my iPod? Turning up the signal from my iPod made it sound better on my stereo. So by not attenuating my signal so much, there is less degradation in the same signal on the screen. Once again, my settings will give me the same 220MHz and 9 effective bits. And once again the scope reads 200mV/div. But in this case, the scope is really set to 100mV/div. Using the TPP0502, I can go down to 2mV/div with absolutely no software zoom (2 x 1mV/div = 2mV/div).</div>
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My peak-to-peak has decreased from 131.4mV to 87.5mV, my RMS has decreased from 18.4mV to 11.5mV. The trace is visibly less noisy. I got about a 35% improvement from probe attenuation alone.</div>
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Also note that the TPP0502 is 2x, but still a full 500MHz. Most of the time, 1x and 2x probes are severely bandwidth limited, and provide only about 25MHz of bandwidth.</div>
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<a href="http://4.bp.blogspot.com/-dmHD6fwAreU/TmAwKw54QmI/AAAAAAAAAb0/y1D_1hti40g/s1600/TPP0502.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="480" src="http://4.bp.blogspot.com/-dmHD6fwAreU/TmAwKw54QmI/AAAAAAAAAb0/y1D_1hti40g/s640/TPP0502.png" width="640" /></a></div>
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So when you want to pick a probe, make sure that the attenuation is as small as possible while still capturing your signal of interest.<br />
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<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right; margin-left: 1em; text-align: right;"><tbody>
<tr><td style="text-align: center;"><a href="http://www.trekequipment.com/onlinestore/new_product_images/LeCroy_PPE2KV.jpg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" height="200" src="http://www.trekequipment.com/onlinestore/new_product_images/LeCroy_PPE2KV.jpg" width="200" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">LeCroy PPE2KV</td></tr>
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For instance, suppose I want to probe a 640V signal with a passive probe. A typical ~1kV passive probe like the <a href="http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&ci=13471&cs=psu&lc=EN">Tektronix P5100A</a>, the <a href="http://www.home.agilent.com/agilent/product.jspx?nid=-34025.536879784.00&cc=US&lc=eng">Agilent 10076A</a>, or the <a href="http://www.lecroy.com/options/default.aspx?categoryid=3&groupid=10&capid=102&mid=508">LeCroy PPE series</a> will have a 100x attenuation. That means that before the oscilloscope has a chance to digitize the signal, it is reduced by a factor of 100x. Suppose I want to measure ripple on my supply? The ideal probe will not only have the right voltage and right bandwidth, but also the least attenuation. The best car stereo system in the world can't improve the sound from an iPod at low volume. Likewise, having more real or effective bits doesn't help if the signal is too attenuated.<br />
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<tr><td style="text-align: center;"><a href="http://www.eetimes.com/ContentEETimes/Images/Products/TandM2/2011-03-07_crh_tekrpobes.jpg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" height="243" src="http://www.eetimes.com/ContentEETimes/Images/Products/TandM2/2011-03-07_crh_tekrpobes.jpg" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Tektronix TPP0850</td></tr>
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For these signals, I'd want to find a probe with more than 10x attenuation, but less than 100x. One example would be the <a href="http://www2.tek.com/cmsreplive/psrep/13471/56W_10262_7_2011.05.05.15.41.15_13471_EN.pdf">Tektronix TPP0850</a> with 50x of attenuation, or even the <a href="http://www2.tek.com/cmsreplive/psrep/13471/56W_10262_7_2011.05.05.15.41.15_13471_EN.pdf">P5120</a> with 20x of attenuation. When I talk about high-bandwidth probes, the typical trade-off is between <span class="Apple-style-span" style="color: #cc0000;">voltage </span>and <span class="Apple-style-span" style="background-color: white;"><span class="Apple-style-span" style="color: #38761d;">bandwidth</span></span>. For power probing, you also have to consider the tradeoff between <span class="Apple-style-span" style="color: #0b5394;">attenuation </span>and <span class="Apple-style-span" style="color: #38761d;">bandwidth</span>.. The P5120 is 200MHz, but only 20x. The TPP0850 is 800Mz and 50x. Either one would do a superior job than a 100x probe in evaluating a power supply ripple.<br />
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<tr><td class="tr-caption" style="text-align: center;">Tektronix P52xx Series</td></tr>
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The same logic applies when looking at high-voltage differential probes. When working in high-voltage systems, you often do not want the ground of your oscilloscope connected to the ground of your circuit. It creates a ground loop that can stop your circuit from functioning properly, and the excessive return current into the oscilloscope ground could damage it. And please never "float" your oscilloscope with a cheater plug that has no ground pin - it can be dangerous and destroy your precious investment.<br />
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In these designs, you should not use a passive high voltage probe, but rather an active high-voltage differential probe. These probes isolate the ground of the oscilloscope away from the ground of the measurement. Many of these probes have switchable attenuation, so pick the one with switches that closely match your design parameters. The <a href="http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&ci=13415&cs=psu&lc=EN">P5202A</a> is 20x or 200x, the<a href="http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&ci=13415&cs=psu&lc=EN"> P5205A</a> is 50x or 100x, and the <a href="http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&ci=13415&cs=psu&lc=EN">P5210A</a> is 100x or 1000x.<br />
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Hopefully that gives you a little better perspective on picking a probe. You want to be sure you pick the probe with the lowest attenuation possible that does not sacrifice the voltage and bandwidth requirements of your signal.Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com1tag:blogger.com,1999:blog-1565037239206599239.post-19579418376590855992011-08-31T23:52:00.004-04:002011-12-06T09:55:59.061-05:00Radar Analysis with the Tektronix MDO4104-6This week, <a href="http://www.tektronix.com/" target="_blank">Tektronix </a>introduced the "<a href="http://www.scoperevolution.com/" target="_blank">scope revolution</a>", the new <a href="http://www.tek.com/products/oscilloscopes/mdo4000/" target="_blank">Mixed Domain Oscilloscope</a>. There is a ton of material already available describing the architecture of the oscilloscope and how it can be used for some common applications, such as <a href="http://www.scoperevolution.com/us/launch/applications/" target="_blank">Zigbee, power supply design, PLL settling</a>, etc. I won't repeat today what you'll find on the Tektronix site. I want to blog on some less common applications. In this case, I am using the MDO4104-6 to analyze a wideband chirped radar.<br />
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<a href="http://1.bp.blogspot.com/-HP-NLW8eKI4/Tl7sdU3mDyI/AAAAAAAAAbg/zrR54iZAhSs/s1600/Tek-mdo4104-6_11a_h.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="295" src="http://1.bp.blogspot.com/-HP-NLW8eKI4/Tl7sdU3mDyI/AAAAAAAAAbg/zrR54iZAhSs/s400/Tek-mdo4104-6_11a_h.jpg" width="400" /></a></div>
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What are some common measurements I need to make in radar design, what tools are used today, and why would I use an MDO to analyze my radar?</div>
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<a href="http://4.bp.blogspot.com/-DnSRMSk_XI0/Tl7y435x3XI/AAAAAAAAAbk/AcngZc8kIa4/s1600/FighterJet.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="138" src="http://4.bp.blogspot.com/-DnSRMSk_XI0/Tl7y435x3XI/AAAAAAAAAbk/AcngZc8kIa4/s200/FighterJet.jpg" width="200" /></a>At its simplest, a <a href="http://www.tek.com/applications/defense_electronics/radar-test.html" target="_blank">radar</a> is a single tone that is sent out as an RF burst. A classic problem in radar design is the tradeoff between range and resolution. In short, a signal with a wide pulse width contains enough RF energy that it is able to travel a long distance to a target. The problem is that if there are multiple targets, their individual reflections will be obscured by the long pulse. Two airplanes will look like one. In other words, long range, poor resolution. </div>
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If you narrow the pulse width, the resolution will improve, but the range will get shorter. In other words, you can discern two airplanes next to each other, but you might miss an airplane too far away.</div>
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The solution is pulse compression. Make the pulse wide, but do something unique in the pulse that enables you to discern multiple reflections. One of the most common techniques is linear frequency modulation. If the pulse is centered at 4GHz, instead of playing out a pure 4 GHz, start the signal at 3.97GHz and sweep to 4.03GHz. Many radar systems use narrow chirps, on the order of 10MHz to 50MHz wide. But to get better spatial resolution, you chirp wider. Modern radars can have chirp bandwidths of 500MHz and beyond!</div>
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<a href="http://www2.tek.com/cmswpt/tidetails.lotr?ct=TI&cs=Primer&ci=14783&lc=EN" target="_blank">How do you test a radar?</a> Often individual components (transmit/receive modules, antenna components, etc) can be tested as part of a golden radar, installed with known good components and evaluated. However, using a device to test itself is not often ideal, so off-the-shelf tools have radar personalities for doing performance evaluation.</div>
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<a href="http://3.bp.blogspot.com/-dvkRn2Xo2eI/TYyn4GN82ZI/AAAAAAAAASo/JZMQSWqTTls/s1600/Tek-rsa5106a_06a_h.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="141" src="http://3.bp.blogspot.com/-dvkRn2Xo2eI/TYyn4GN82ZI/AAAAAAAAASo/JZMQSWqTTls/s200/Tek-rsa5106a_06a_h.jpg" width="200" /></a>For radars with bandwidths under 100MHz, a spectrum analyzer can be used to measure the characteristics. Tektronix, Rohde, and Agilent all make digitizing spectrum analyzers that can downconvert an RF pulse to baseband and store it as digital I and Q. From that data, measurements can be made. The <a href="http://www.tek.com/products/spectrum-analyzer/rsa6100a/" target="_blank">Tektronix RSA6000</a> and <a href="http://www.tek.com/products/spectrum-analyzer/rsa5000/" target="_blank">RSA5000</a> series can perform 27 different scalar and vector pulse measurements. Scalar means amplitude-vs-time based measurements, such as the pulse risetime, width, duty cycle, ripple, droop, etc. Vector means measurements like frequency deviation, frequency error, impulse response, phase deviation, and pulse-to-pulse phase error.</div>
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<a href="http://1.bp.blogspot.com/-DQU6cNNwZks/TYZWIuQL15I/AAAAAAAAAPk/tmkOk24NTEY/s1600/mso72004c_small.JPG" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="146" src="http://1.bp.blogspot.com/-DQU6cNNwZks/TYZWIuQL15I/AAAAAAAAAPk/tmkOk24NTEY/s200/mso72004c_small.JPG" width="200" /></a>For radars with wider bandwidths, a <a href="http://www.tek.com/products/oscilloscopes/dpo70000_dsa70000/" target="_blank">real-time digital oscilloscope is typically used to acquire the signal</a>. There are oscilloscopes with enough sample rate to digitize the RF carrier from vendors like Tektronix, Agilent, and LeCroy. You just buy enough bandwidth to digitize your signal. With <a href="http://www.tek.com/products/oscilloscopes/dpo70000_dsa70000/" target="_blank">oscilloscopes going in real-time bandwidth beyond 30GHz today</a>, just about any signal can be acquired and analyzed. Combined with specialized software packages (such as <a href="http://www.tek.com/products/accessories/application_software/signalvu.html" target="_blank">Tektronix SignalVu</a>), the same 27 vector and scalar measurements can be made.<br />
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The world seems happy with golden radars, spectrum analyzers, and oscilloscopes, so where does the <a href="http://www.tek.com/products/oscilloscopes/mdo4000/" target="_blank">Mixed Domain Oscilloscope</a> fit for radar diagnostics?</div>
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<li>Portability and size. At just 5" deep and 11lbs, it is far easier to carry than larger instruments. The aforementioned spectrum analyzers and oscilloscopes can weight over 50 lbs and can be the size of a small piece of luggage.</li>
<li>Cost is considerably less. Often a lab can only afford one high-end spectrum analyzer or high-bandwidth scope. Each of these may cost over $100k. But at less than $30k, every bench can have an MDO.</li>
<li>Most importantly, the MDO gives users the unique ability to not only analyze their radar signal, but also the other numerous digital gates and timing signals present in a radar system.</li>
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The MDO has 4 analog channels with bandwidths up to 1GHz, 16 digital channels, and 1 RF input. The RF input can be tuned up to 6GHz with a minimum real-time bandwidth of 1GHz. It can also decode up to 4 buses, such as <a href="http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=17882&lc=EN" target="_blank">MIL-STD-1553 and RS-232</a>.</div>
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Bear in mind that the MDO <b><u>does not</u></b> replace an oscilloscope with a dedicated radar analysis package like SignalVu. Examples of this type of oscilloscope include the Tektronix DPO/MSO5000, 7000, and 70000. <a href="http://www2.tek.com/cmswpt/tidetails.lotr?ct=TI&cs=Application+Note&ci=15006&lc=EN" target="_blank">SignalVu can do significantly more measurements specifically related to pulsed radar</a>, including pulse-to-pulse frequency difference, pulse-to-pulse phase difference, frequency error over the pulse, impulse response, and can analyze up to 10,000 consecutive pulses. This powerful, detailed analysis is not replicated on the MDO, so both tools really have a different role to play in an RF tool bench. Below is an example from a scope running SignalVu, analyzing consecutive pulse parameters, showing a histogram of those parameters across up to 10,000 pulses, and an impulse-response spectrum from the chirp.</div>
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<tr><td class="tr-caption" style="text-align: center;">SignalVu on a High Bandwidth Scope - Tremendous Single Channel Analysis of a Radar Signal</td></tr>
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The MDO does a subset of the analysis you can do with a scope running SignalVu, but an important subset that often covers the basic need, while adding the functionality of multiple channel time correlation.</div>
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<tr><td style="text-align: center;"><a href="http://3.bp.blogspot.com/-4sq8SlwV1Nc/Tl7sb4pIJ2I/AAAAAAAAAbY/HETsirAYLlk/s1600/Tek-mdo4104-6-360_01.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="228" src="http://3.bp.blogspot.com/-4sq8SlwV1Nc/Tl7sb4pIJ2I/AAAAAAAAAbY/HETsirAYLlk/s400/Tek-mdo4104-6-360_01.jpg" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Tektronix MDO4104-6</td></tr>
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In the following display, I have quite a few traces. Starting at the top, I have the amplitude vs time of the radar pulse centered at <span class="Apple-style-span" style="color: red;">4GHz with a 500MHz</span> bandwidth. This would be similar to "zero span" on a conventional spectrum analyzer, although no spectrum analyzer could do a 500MHz zero span display. It is labeled "A" for "Amplitude vs Time". The next trace down is labeled "f" for "Frequency Vs Time". It shows the frequency deviation during the pulse. The trace is blank in spots because I have squelched the frequency display when the pulse is not on.</div>
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Below that signal is the trigger for the transmit pulse (<span class="Apple-style-span" style="color: #bf9000;">channel 1</span>), a digital gate that is valid during the pulse (<span class="Apple-style-span" style="color: cyan;">channel 2</span>), a receive trigger for the return pulse (<span class="Apple-style-span" style="color: magenta;">channel 3</span>), and a system clock for all of my digital gates (<span class="Apple-style-span" style="color: lime;">channel 4</span>). In the interest of saving space, I am not showing them, but there are 16 more digital channels I can assign to gates in my system.</div>
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<a href="http://4.bp.blogspot.com/-yz6Er1eIMRU/Tl7pr82qsBI/AAAAAAAAAbU/B5sH6OQpKvk/s1600/MDO-Radar-TimeDomainOnly.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="480" src="http://4.bp.blogspot.com/-yz6Er1eIMRU/Tl7pr82qsBI/AAAAAAAAAbU/B5sH6OQpKvk/s640/MDO-Radar-TimeDomainOnly.png" width="640" /></a><br />
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I have triggered on the <span class="Apple-style-span" style="color: #b45f06;">RF burst</span> (see the RF trigger at -9dBm in the lower right), and made a series of automatic measurements. I have measured the risetime (<i><b>4.957us</b></i>), pulse width (<i><b>31.35us</b></i>), duty cycle (<i><b>18.8%</b></i>), and PRF (<b><i>5.99kHz</i></b>) of the radar. I have also measured the delay from the TX trigger rising edge to the start of the envelope (<b><i>15.15us</i></b>), and the delay from the falling edge of the RF pulse to the rising edge of the receive gate (<b><i>7.773us</i></b>). There is also a Peak-Peak measurement on the Frequency Vs Time, showing a <b><i>504MHz</i></b> frequency deviation on the chirp. Finally, I have measured my system clock which is running at <b><i>120MHz</i></b>.<br />
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Also, note that while I triggered on the rising edge of the RF burst, the MDO can also trigger on other pulse characteristics as well. It can be setup to trigger on narrow pulses, wide pulses, runt pulses (pulses that are not full amplitude), and even missing pulses. You could even setup to trigger when the RF pulse is present but the On-Gate was not (using a logic trigger). No regular oscilloscope or spectrum analyzer can trigger on all of these conditions.</div>
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Below, I am showing the same data but I have split the screen to also show the RF spectrum of a pulse. You can see the orange bar under the second pulse indicates where I am doing my FFT. To get a good looking FFT of a chirped radar, you need to set the window function to Rectangular as Kaiser will just look like a series of spikes in the frequency domain. With this display up, I am using manual markers to measure occupied bandwidth. From 4.235 to 3.735, I have my 500MHz. I can also move the spectrum analysis window to show the pulse off time, seeing what the spectrum looks like when the pulse is not active.<br />
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<a href="http://3.bp.blogspot.com/-HtqDqZj3qRI/Tl7pr7ovR7I/AAAAAAAAAbQ/CsiiMJR8nng/s1600/MDO-Radar-TimeAndFreqDomain.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="480" src="http://3.bp.blogspot.com/-HtqDqZj3qRI/Tl7pr7ovR7I/AAAAAAAAAbQ/CsiiMJR8nng/s640/MDO-Radar-TimeAndFreqDomain.png" width="640" /></a></div>
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The ability to capture high frequency RF and slower control signals is unique to an MDO. Suppose I was analyzing a narrowband radar at 4GHz. A spectrum analyzer could do a tremendous amount of analysis, but could not acquire other related signals like the digital gates. A real-time oscilloscope could acquire my RF and additional signals, but would be limited in capture depth. If my real-time oscilloscope digitizes at 50GS/s and comes standard with 10M points of memory, it can only capture 200us of data. Even at max memory, it can still only capture 5ms of RF data. If I lower the sample rate, I can capture more time, but I won't be sampling high enough to get my RF data. How do I see a 4GHz signal and a 100MHz clock?<br />
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While the MDO4104-6 time correlates RF and analog data, the digitizers are independent. So while the RF section captures a pulse at 4GHz (storing up to 75ms of RF Data), the analog section can run at 500MS/s and with just 20M of memory store about the same record.<br />
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Mixed Domain Oscilloscopes are a pretty new and exciting category. They certainly do not replace spectrum analyzers or conventional digital oscilloscopes for radar analysis. In my mind, the MDO is a tool that overlaps some key functions at a smaller size/lower price point, while simultaneously adding key functionality not available on any other platform.Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com2tag:blogger.com,1999:blog-1565037239206599239.post-61432649733606711782011-08-22T20:53:00.006-04:002011-08-25T20:13:56.398-04:00To run Windows or not to run Windows - All About Oscilloscope Operating SystemsQuick.... look at both of these <a href="http://www.tek.com/products/oscilloscopes/">oscilloscopes</a>. What's the difference between them?<br />
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<div class="separator" style="clear: both; text-align: center;"><a href="http://1.bp.blogspot.com/-lnREnmIEe_k/TlLw6tBm3_I/AAAAAAAAAbE/jZyx7OD-o34/s1600/Tek-DPO4104b_01b_h.jpg" imageanchor="1"><img border="0" height="238" qaa="true" src="http://1.bp.blogspot.com/-lnREnmIEe_k/TlLw6tBm3_I/AAAAAAAAAbE/jZyx7OD-o34/s400/Tek-DPO4104b_01b_h.jpg" width="400" /></a></div> <br />
<div class="separator" style="clear: both; text-align: center;"><a href="http://3.bp.blogspot.com/-XZsfDkggj4A/TlLw61F-vgI/AAAAAAAAAbI/rh58LcK1ZW4/s1600/Tek-DPO5204_01b_h.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="237" qaa="true" src="http://3.bp.blogspot.com/-XZsfDkggj4A/TlLw61F-vgI/AAAAAAAAAbI/rh58LcK1ZW4/s400/Tek-DPO5204_01b_h.jpg" width="400" /></a></div>Both oscilloscopes look awfully similar. They are both the same height, width, and have the same screen size. They have the same button layout, and both are available in 350MHz, 500MHz, and 1GHz variants. What's the difference?<br />
<a name='more'></a>The oscilloscope on the top is a <a href="http://www.tek.com/products/oscilloscopes/mso4000/">Tektronix DPO4104B</a>, an oscilloscope that uses an embedded Linux operating system, invisible to the user. The oscilloscope on the bottom is a <a href="http://www.tek.com/products/oscilloscopes/mso5000/">Tektronix DPO5204</a>, running Microsoft Windows 7 Ultimate 64-bit.<br />
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So why do some oscilloscopes run Windows and others run embedded operating systems that are closed?<br />
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There is the simple answer. Lower bandwidth oscilloscopes don't use Windows because the hardware to run it and the license fees to Bill Gates would drive up the cost too much. Nobody wonders why a $990 <a href="http://www.tek.com/products/oscilloscopes/tds2000/">TDS2000C</a> doesn't run Windows.<br />
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Additionally, the processing power required to run a full load of Windows likely makes the scope bigger and heavier (and take longer to boot), things you do not want in a small bench scope. The DPO5000 weighs a bit more and is deeper than the DPO4000 (though they look the same from the front). And as we will discuss, non-Windows oscilloscopes tend to be simpler from a maintenance point of view, a bonus for a scope that might be widely distributed among different users.<br />
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On the high end, everybody's oscilloscope runs Windows. Once you reach 2GHz and above, Windows can be assumed. Engineers always want to know why can't Tektronix or Agilent or LeCroy make performance oscilloscopes with embedded closed operating systems, or scopes that run MacOS, Android, or anything but Windows?<br />
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The simple answer is that as applications get more complex (<a href="http://www.tek.com/applications/computing/serial-communications.html">PCI-Express, Superspeed USB, XAUI, SATA, SFP+, 100G Ethernet, DQPSK, Wideband Radar, etc</a>), the market of users gets smaller. So much work and development goes into the algorithms for waveform analysis that it makes sense to use the most common environment tools available, such as .NET and <a href="http://www.mathworks.com/tektronix">MATLAB</a>. The engineering effort to create a high bandwidth oscilloscope that did not run Windows would not be economically viable at this point today.<br />
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<span lang="">Looking at the 4 major vendors of Mid-Range oscilloscopes, you can get into a Windows oscilloscope at the 200MHz performance point (LeCroy WaveSurfer 24MXs-B), 350MHz (Tektronix DPO5034), 600MHz (Agilent DSO9064), or 1GHz (Rohde RTO1004). You can can stay with an embedded non-Windows OS until you get above 1GHz (Tektronix DPO4104B, Agilent DSO7104B, Rohde RTO1024).</span><br />
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In the graph below, the top 4 lines are non-Windows oscilloscope bandwidth coverage (Blue-Tektronix, Red-Agilent, Green-LeCroy, Purple-Rohde), and the bottom 4 lines are Windows bandwidth coverage. Tektronix seems to have the most complete coverage in the "overlap" region, with a dual product line from 350MHz to 1GHz, the widest of the field.<br />
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<div class="separator" style="clear: both; text-align: center;"><a href="http://4.bp.blogspot.com/-eQZZFzXCMzM/TlKM4nmVrQI/AAAAAAAAAao/xY_KUISNgbo/s1600/WindowsChart.png" imageanchor="1" style="clear: left; cssfloat: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="388" qaa="true" src="http://4.bp.blogspot.com/-eQZZFzXCMzM/TlKM4nmVrQI/AAAAAAAAAao/xY_KUISNgbo/s640/WindowsChart.png" width="640" /></a></div><br />
So if I want a low bandwidth scope (below 200MHz), it must be non-Windows. If I want a high-bandwidth scope (>1GHz), it must be Windows. These decisions are made by the market today based on what is available. But from 200MHz to 1GHz, how do I decide?<br />
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The first decision might be application based. Some oscilloscope vendors have written test applications geared around a particular platform. For instance, if you to do full Ethernet compliance test with <a href="http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=17771&lc=EN">Tektronix TDSET3</a>, while a 1GHz oscilloscope is sufficient for debug and decode, you need the DPO5104 Windows based oscilloscope for full compliance. The application is only written for the Windows environment.<br />
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Other applications are written for both environments. For example, there is a need for power supply testing in both platforms, so there are tools like <a href="http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=18343&lc=EN">DPO4PWR</a> for the DPO4000 and <a href="http://www.tek.com/applications/design_analysis/power-analysis.html">DPOPWR</a> for the DPO5000. The same algorithms were rewritten for both platforms. The Windows variant has some extra analysis features not found on the non-Windows platform, but both are essentially the same.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://2.bp.blogspot.com/-_qi43Py772A/TlKM5bFvCcI/AAAAAAAAAaw/7KBNXo142iI/s1600/DPO4PWR-SwitchingLoss.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="239" qaa="true" src="http://2.bp.blogspot.com/-_qi43Py772A/TlKM5bFvCcI/AAAAAAAAAaw/7KBNXo142iI/s320/DPO4PWR-SwitchingLoss.png" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">DPO4000 showing DPO4PWR - NonWindows</td></tr>
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<tr><td style="text-align: center;"><a href="http://4.bp.blogspot.com/-7GRjhNwU88U/TlKM5CasgDI/AAAAAAAAAas/uNHdg7P5pM4/s1600/DPOPWR-SwitchingLoss.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="240" qaa="true" src="http://4.bp.blogspot.com/-7GRjhNwU88U/TlKM5CasgDI/AAAAAAAAAas/uNHdg7P5pM4/s320/DPOPWR-SwitchingLoss.png" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">DPO5000 showing DPOPWR - Windows 7 64-bit Ultimate</td></tr>
</tbody></table> So if my choice is not made by bandwidth, and my choice is not made by application, what else should I consider? Often the Windows version of an oscilloscope will have more advanced features than a non-Windows version of the same bandwidth. For instance, look for extra memory depth for capturing long waveforms, extended trigger capabilities, and additional math and measurement functionality that a vendor may choose to put on a Windows oscilloscope. Many users want to run programs like MATLAB on the oscilloscope itself and stream data directly into it, so they choose a Windows product.<br />
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Many choose Windows based oscilloscopes for their connectivity, from support for thumb drives, Ethernet connectivity, and network printers. However, even non-Windows oscilloscopes like the DPO4000 now support all of these features.<br />
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There are a few cautions when dealing with a Windows oscilloscope. If you treat it like a scope and never touch the IO ports (USB, Ethernet), you could technically use it as a scope for the rest of your life and never care that it runs Windows. If it works today, it will work 10 years from now. I still see some Windows 98 scopes in use out there.<br />
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The problem in the real world is that somebody is going to stick a thumb drive into the scope and somebody else will want it on the network. Instantly you are subject to the needs for corporate security, user accounts, firewalls, virus scanners, spyware blockers, etc. It is important to be sure that the oscilloscope still runs after your IT department is through with it!<br />
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Many vendors publish a document describing the best ways to handle security on a Windows based oscilloscope. <a href="http://www.tek.com/applications/defense_electronics/pdf/37W-26284-0.pdf"> Here is an example of a document published by Tektronix on configuring Windows products.</a><br />
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You should also look at the underlying PC of a Windows oscilloscope. Some vendors tell you the minimum PC performance in the oscilloscope, and others do not specify it at all. If the company cannot tell you what kind of PC you will get, this is a red flag to avoid the product. Similarly, you don't want to spend $10k-$30k and find your product obsolete in 3 years, so I would personally think carefully before buying a product still running Windows XP. I don't know how long the security support will continue to exist for Windows XP, but for sure Windows 7 will run longer.<br />
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For simplicity and ease of use, many oscilloscope users prefer the closed operating systems. As a result, we are seeing an industry trend towards more advanced "bench" scopes, where application specific software is being written to run on bench instruments. A Tektronix MSO4104B has power testing, mask testing, basic jitter measurement, screen histograms, 1M point FFT's, tons of measurements, math equation editor, and tons of other features that people think require a Windows oscilloscope. Yet since it does not run Windows, it is a smaller scope that boots faster and has less security issues.<br />
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For the record, I enjoy both tools. If I had to pick only 1, I would have to go with the Windows based oscilloscopes merely because that's where all of the heavy duty analysis tools exist. But for general debug, I still prefer my Tektronix MSO4104B, a great scope that does not run Windows.Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com2tag:blogger.com,1999:blog-1565037239206599239.post-42236118864083937922011-08-10T00:17:00.000-04:002011-08-10T00:17:33.259-04:00Oscilloscope Revolution - Analog + Digital + RFThis is a technology blog, but this news is so big, I thought I should use my blog to help promote it. Besides, sign up for the launch party and you can win one of $30,000 worth of prizes! I guarantee you won't want to miss August 30, 2011!<div><br />
</div><div>Check it out and sign up now:</div><div><a href="http://www.scoperevolution.com/">http://www.scoperevolution.com</a></div><div><br />
</div><div><br />
</div>Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com1tag:blogger.com,1999:blog-1565037239206599239.post-10343890624412980232011-08-08T00:43:00.006-04:002011-08-25T20:14:28.665-04:00What's the difference between a GHz and a GS?One of the most confusing things for real-time oscilloscope buyers is the difference between sample rate and bandwidth. I remember when I started with Tektronix, I heard people tell me they owned a "20GHz Agilent oscilloscope". At the time, Tektronix was the only one making a 20GHz oscilloscope. Agilent made nothing more than 13GHz. What they were telling me was impossible. However, the lower bandwidth Agilent oscilloscopes did have 20GS/s sample rate, which was the source of their confusion.<br />
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So what is the difference between a GHz and a GS?<br />
<a name='more'></a>In short, gigahertz (GHz) is a measure of the frequency content that the scope can see, and gigasamples per second (GS/s) is a measure of the digitizing rate of the scope.<br />
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On the GS side, the bandwidth of the oscilloscope is determined by the Nyquist bandwidth, or the requirement that the oscilloscope must digitize at least 2x higher than the highest frequency content in the signal. Typically, oscilloscope vendors use the 2.5x rule of thumb for specifying sample rate. Some optical and RF applications are okay with 2x, and some high speed serial applications require more than 2.5x. But 2.5x is a pretty good industry standard. So if you have a 2GHz oscilloscope, then it requires 5GS/s of sampling rate.<br />
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So why does the Tektronix DPO73304D with 100GS/s not have a 50GHz bandwidth? Why does a Tektronix MSO4104B with 5GS/s not have 2.5GHz of bandwidth?<br />
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In the following example, you can see a generic 2GHz oscilloscope with a 5GS/s sampling rate. This is a generic example and not matched to anything in the market today. The first piece of the oscilloscope that determines the bandwidth is actually the variable gain front-end amplifier. This is the part that the volts/division knob adjusts when you change vertical scale. It also acts like a low pass filter, passing only the specified signal through at the specified amplification to fill the digitizer input.<br />
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<div class="separator" style="clear: both; text-align: center;"><a href="http://1.bp.blogspot.com/-5lOiUE2EPYQ/Tj9fKEYcx-I/AAAAAAAAAZ0/jkSIxE5xYwM/s1600/2GHzScope.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="107" src="http://1.bp.blogspot.com/-5lOiUE2EPYQ/Tj9fKEYcx-I/AAAAAAAAAZ0/jkSIxE5xYwM/s400/2GHzScope.jpg" t$="true" width="400" /></a></div><br />
Now suppose I only had a 1GS/s digitizer. The 2.5x Nyquist rule tells me I can only digitize a 1GS/2.5 or 400MHz signal with this digitizer. The solution is to use a track-and-hold as a decelerator. In the above case, the track-and-hold reduces my 2GHz signal by a factor of 5, delivering it to five slower digitizers. The effective sampling rate of my system is 5GS/s, and the bandwidth is determined by the front-end and track-and-hold combination.<br />
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If I wanted a 4GHz oscilloscope, I could use the exact same digitizers as above. I would just need a higher-bandwidth front-end with a higher speed track-and-hold.<br />
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<div class="separator" style="clear: both; text-align: center;"><a href="http://4.bp.blogspot.com/-7C_BTT5c4M8/Tj9fKBXqzlI/AAAAAAAAAZw/TZjfPmdhp1o/s1600/4GHzScope.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="186" src="http://4.bp.blogspot.com/-7C_BTT5c4M8/Tj9fKBXqzlI/AAAAAAAAAZw/TZjfPmdhp1o/s400/4GHzScope.jpg" t$="true" width="400" /></a></div>In this example, the same base digitizer now yields a 10GS/s system, with a 4GHz bandwidth. In both examples, I actually use the same speed digitizer. But the track-and-hold means my system digitizing rate is twice as high in the second example.<br />
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<div class="separator" style="clear: both; text-align: center;"><a href="http://4.bp.blogspot.com/-tc-4eXej54s/Tj9p5TPGpbI/AAAAAAAAAZ4/UgdDHV52V00/s1600/2ghz-10gs.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="187" src="http://4.bp.blogspot.com/-tc-4eXej54s/Tj9p5TPGpbI/AAAAAAAAAZ4/UgdDHV52V00/s400/2ghz-10gs.jpg" t$="true" width="400" /></a></div><br />
Now if I were to put this same track-and-hold on the 2GHz front end (10GS with 2GHz), my system would still only be 2GHz. I may get some advantage to oversampling in noise reduction or effective number of bits improvement, but I do not get extra bandwidth. The lowest bandwidth piece of the link determines my eventual system bandwidth.<br />
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There is a cool trick to determine the base rate of any digitizer system. FFT the trace and apply averaging to remove random noise. In both cases above, you would see spikes every 1GHz, indicating that despite being 5GS or 10GS systems, there is interleaving being performed. The spikes are called interleave errors, and while major oscilloscope vendors work hard to reduce them, they will always be present. Even femtoseconds of error will appear in an FFT since they are consistent and repeated.<br />
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In the past year, both major oscilloscope manufacturers announced high-bandwidth oscilloscopes. Agilent introduced their oscilloscope based on Indium Phosphide (InP) technology, a 32GHz, 80GS/s oscilloscope, the DSOX93204A. Tektronix introduced an oscilloscope based on 8HP Silicon Germanium (SiGe), a 33GHz, 100GS/s oscilloscope. That oscilloscope is the Tektronix DPO73304D.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://4.bp.blogspot.com/-g48OqSn2_Nc/Tj_MA5yX8ZI/AAAAAAAAAaA/0bq83lD_LQQ/s1600/1108_Tektronix_DPO73304D.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="253" src="http://4.bp.blogspot.com/-g48OqSn2_Nc/Tj_MA5yX8ZI/AAAAAAAAAaA/0bq83lD_LQQ/s400/1108_Tektronix_DPO73304D.jpg" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Tektronix DPO73304D - 33GHz and 100GS/s Real-Time Oscilloscope</td></tr>
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Reading press releases from both companies, they stress that the new chipset replaces the front-end and track-and-hold, but neither makes any claim as to new digitizers. As you can see above, the bandwidth is determined by the front-end and track-and-hold, not the digitizer. In fact, a simple FFT on the Agilent DSOX93204A reveals they are still using their 250MS/s CMOS digitizer, and Tektronix is using the same 3.125GS digitizers in their DPO73304D. Now there are distinct advantages to using a higher base-rate digitizer, including less digitizer distortion and better spurious response. But the base-rate of the digitizer does not determine the overall system bandwidth.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://2.bp.blogspot.com/-G1dfifKSnfY/Tj_LrODYu5I/AAAAAAAAAZ8/Otw-HmDW0wI/s1600/TektronixFrontEnd.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="266" src="http://2.bp.blogspot.com/-G1dfifKSnfY/Tj_LrODYu5I/AAAAAAAAAZ8/Otw-HmDW0wI/s400/TektronixFrontEnd.jpg" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Tektronix SiGe 8HP - 100GS Track-and-Hold (center) with two 33GHz front-ends (sides)</td></tr>
</tbody></table>In the above photo, you can see there are 100GHz SMPM connectors directly on the package of the ASIC that contains both the track-and-hold and the front-end. In fact, there are 2 front-ends on the same multi-chip module, so the 100GS/s track-and-hold can supply 100GS to a single channel or 50GS to two channels. The connectors are on the package itself because the signal is high bandwidth until it hits the track-and-hold, so the desire is to preserve every bit of signal strength as long as possible. Once it passes by the track-and-hold, it is decelerated and can be sent over the PCB to be digitized.<br />
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The track and hold itself is an 8-way track and hold, so it can split a single 100GS signal 8-ways or two 50GS signals 4-ways (you can see those traces above and below the package). This feeds the 12.5GS digitizer module, which also has a slower track-and-hold on-board that gets down to the 3.125GS base-rate.<br />
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For more on the acquisition technology, watch this video: <a href="http://www2.tek.com/cmswpt/pidetails.lotr?ct=PI&cs=wbn&ci=18809&lc=EN">How the Tektronix DPO73304D acquisition system operates</a>.<br />
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The other reason it is important to understand sample rate is that both the Tektronix and Agilent oscilloscopes divide sample rate. On the Tektronix DPO73304D, it is 100GS/s on 2 channels. Pure Nyquist rules would indicate effective bandwidth of 40-50GHz, but the front-end limits it to 33GHz. The extra oversampling serves to reduce noise and improve effective bits. When going to 4 channels, the oscilloscope provides 50GS/s, which is 20-25GHz depending on the oversampling needed for a given application. <br />
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On the Agilent DSOX93204A, it is only 80GS/s on 2 channels, which is sufficient at 2.5x for a 32GHz system, but does not give you any extra oversampling. On 4 channels, it is 40GS/s, which at 2.5x Nyquist yields 16GHz of bandwidth.<br />
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There is a way to get the full 33GHz on all 4 channels of the Tektronix DPO73304D, and that is to use equivalent time sampling. I will discuss exactly what this means in a later post, but the DPO73304D can achieve 10TS/s so long as a signal is repetitive and there is a trigger. Now 10TS/s does not provide 4THz of bandwidth, because the front-end will still limit the system to 33GHz. But it does provide plenty of bandwidth to ensure 33GHz on all 4 channels simultaneously.Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com3tag:blogger.com,1999:blog-1565037239206599239.post-17231045090245309702011-07-28T11:55:00.006-04:002011-08-25T20:15:06.501-04:00How to accurately measure vertical noise on a real-time oscilloscopeMy introduction to the world of being a field applications engineer was a frantic call from a sales person. His customer had been told that a competitor's oscilloscope was "lower noise" due to Faraday cage shielding, something that presumably my product did not have. Now, I recognize marketing hype when I hear it, but I had to understand the misperception in detail to explain it to the customer. Every scope has "faraday cage shielding" - it's a standard part of designing high bandwidth front-ends. Michael Faraday died in 1867, so it amazed me that anybody would act like using a metal shield was something revolutionary, but here it was in marketing literature.<br />
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At first I suspected the customer was concerned a small difference in noise that really didn't matter. But when the customer said we had twice the noise of our competition, I had to understand what was going on. <br />
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How do you measure noise on an oscilloscope?<br />
<a name='more'></a>The following is a brief primer on how to measure noise on a real-time oscilloscope, and the pitfalls when comparing two oscilloscopes next to each other. Whether the noise is on a Tektronix, Agilent, Rohde, or LeCroy oscilloscope, these standard techniques always apply.<br />
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<ol><li><strong><span style="color: blue;">Assess if the noise difference is quantitative or merely visual.</span></strong></li>
<ul><li><span style="font-size: xx-small;">When I first heard the claim that a competitor's scope had lower noise, the competitor's scope at the time had a smaller display that was the same resolution as ours. In other words, the pixels were smaller, so the trace appeared thinner. That was an older model, and they have since introduced a new model with a bigger screen. But still, remember that the scope is attempting to take millions of digitized data points and compress them to display in 1000 pixels. Choosing how to visually decimate the data and how to intensity grade the trace is a tricky process. Some methods may smooth out important data while others may over accentuate noise spikes. The point is that visual presentation is subjective to the decisions of those who raster the trace, and can have little bearing on actual noise present. Noise must be measured, not viewed visually.</span></li>
</ul><li><strong><span style="color: blue;">Ensure both oscilloscopes are properly self-calibrated at temperature.</span></strong></li>
<ul><li><span style="font-size: xx-small;">Modern oscilloscopes make use of interleaved digitizers to achieve full sample rate. A Tektronix DPO72004C interleaves 32 digitizers to get to 100GS/s. An Agilent DSO93204-X interleaves 320 digitizers to get to 80GS/s. Both products have a self-calibration that must be run to align the digitizers. Excess interleave error can make noise higher. A quick check on a Tektronix oscilloscope is to glance in the lower right corner. If you see a yellow or red thermometer, you shouldn't trust the measurements. Let the scope warm up for 30 minutes and run the internal self-calibration.</span></li>
</ul><li><strong><span style="color: blue;">Make sure both oscilloscopes are set to the same bandwidth.</span></strong></li>
<ul><li><span style="font-size: xx-small;">This one should be obvious, but many people miss it. A 20GHz oscilloscope has more noise than a 13GHz oscilloscope because it is integrating noise over a wider bandwidth. Noise comes from many sources in the environment, so the higher the bandwidth oscilloscope, the higher the noise. Most modern oscilloscopes have DSP to limit the bandwidth. If you want to compare a 20GHz scope with a 13GHz scope, use the DSP to limit the 20GHz scope to 13GHz.</span></li>
</ul><li><strong><span style="color: blue;">Make sure both oscilloscopes are set to the same full scale voltage, NOT the same volts per division.</span></strong></li>
<ul><li><span style="font-size: xx-small;">As previously explained </span><a href="http://effectivebits.blogspot.com/2011/03/oscilloscopes-exposed-volts-per.html"><span style="font-size: xx-small;">in my blog post on the volts per division knob</span></a><span style="font-size: xx-small;">, each vendor divides the screen up differerently. Everybody uses an 8-bit digitizer across the screen, but Rohde and Tektronix use 10 divisions, whereas Agilent and Lecroy use 8 divisions. If an Agilent scope is set to 100mV/div, the Tektronix scope must be set to 80mV/div for an equal comparison. The alternative is to divide the measured noise by the full scale voltage to measure noise in percentage. Either way, you cannot compare an absolute noise (measured in microvolts) at a particular V/div setting if the 2 scopes divide the screen up differently. Read the blog post on volts per division for a better explanation.</span></li>
<li><span style="font-size: xx-small;">Suppose scope X has 3mV of RMS noise at 100mV/div, but 8 divisions. Another scope, Scopy Y, has 3.5mV of RMS noise at 100mV/div, but 10 divisions. Which has more noise, scope X or scope Y? At 100mV/div, Scope Y has "more noise". But scope Y has 10 divisions. So the proper way to say it is that at 100mV/div, Scope X has 8 * 100mV or 800mV full scale. 3mV/800mV = 0.375% noise. At 100mV/div, Scopy Y has 10 * 100mV or 1V full scale. 3.5mV/1V = 0.35% noise. Therefore, Scope Y has less noise at 100mV/div even though its noise value initially appears higher.</span></li>
<li><span style="font-size: xx-small;">As you can see, to compare noise between two oscilloscopes on equal volts/division setting, you must use percent of full scale, not absolute voltage. To use absolute voltage, full scale must be the same. In the above case, Scope Y would be set to 80mV/div for an accurate comparison (10 divisions * 80mV/div is the same as 8 divisions * 100mV/div).</span></li>
</ul><li><strong><span style="color: blue;">Do not measure peak-to-peak noise.</span></strong></li>
<ul><li><span style="font-size: xx-small;">It is tempting to put the scope into a continuous run mode and measure noise peak-to-peak. However, this value is known as unbounded because it gets larger the longer you run the measurement. If two scopes have equal noise, the one that processes data faster can appear to have higher peak-to-peak noise than one that processes data slower.</span></li>
</ul><li><strong><span style="color: blue;">Do not use DC-RMS measurement, use AC-RMS.</span></strong></li>
<ul><li><span style="font-size: xx-small;">RMS measurement is statistically valid, and does not suffer from the peak-to-peak issue above. RMS should not change based on the observation period. However, most oscilloscopes have a DC-RMS measurement. The problem with DC-RMS is that we are measuring a miniscule voltage, one that is likely fractions of a percent of the full scale voltage. However, scopes have an offset error that is specified. DC-RMS will include DC offset error. While DC offset error is important to measure, it needs to be excluded from a noise measurement. Some scopes have a checkbox to AC-couple the RMS measurement (Agilent) and others have a special AC-RMS measurement (Tektronix). Either way works, but you need to be aware of it.</span></li>
</ul><li><strong><span style="color: blue;">If AC-RMS is not available, use the standard deviation of a vertical histogram.</span></strong></li>
<ul><li><span style="font-size: xx-small;">The best way to measure noise is to do a histogram of the trace itself in the vertical axis. The histogram will ignore the DC offset of the signal, and the standard deviation of the histogram is the RMS noise. On a Tektronix windows-based scope, just draw a box on the screen and select "Vertical Histogram". Right click on the histogram and measure the standard deviation. On a Tektronix DPO4104B, you can make the histogram measurement under the "Measure" menu.</span></li>
</ul><li><strong><span style="color: blue;">Compare all settings as some will be better than others.</span></strong></li>
<ul><li><span style="font-size: xx-small;">Another danger is to allow a sales person to set 2 oscilloscopes to the strong spot on one scope and the weak spot on another. Any two products will naturally have good spots and weaker spots. You must look at the products on the whole range of settings.</span></li>
</ul><li><strong><span style="color: blue;">Question what design choices impact the vertical noise displayed on a channel.</span></strong></li>
<ul><li><span style="font-size: xx-small;">Two oscilloscopes of identical banner specification may have truly different performance. I will address this in a later blog post, but just because two oscilloscopes say "16GHz", it does not mean they have identical response. By industry definition, a scope that is 3dB down at 16GHz is a 16GHz oscilloscope, even if it was 1-2dB down for half of the bandwidth. This would result in a slower risetime, but would also result in lower noise (since the noise bandwidth at higher frequencies is attenuated). Even past the stated bandwidth, some scopes use DSP to create a brickwall filter, while others have a more natural gaussian roll off. So one 16GHz scope may be 10dB down at 17GHz, and another may be 5dB down at 17GHz. These are design choices that can impact noise. Faster roll-off can mean less noise but can also mean a worse pulse response. When you've accounted for factors 1-8 above, then you must question scope risetime, overshoot, roll-off past bandwidth, and other factors such as ENOB (subject for a later post)</span></li>
</ul><li><strong><span style="color: blue;">Beware of claims of a company - noise specifications are always typical, and not always at every setting.</span></strong></li>
<ul><li><span style="font-size: xx-small;">When you see noise claims on a datasheet, bear in mind that there is generally a footnote indicating noise performance is "typical" and not "specified/guaranteed". These words mean different things to different vendors. For some, it means a number that cannot be traced or guaranteed without adding significant calibration cost. A true guaranteed specification must be met within a certain statistical tolerance in all operating conditions, and often must be verified during production testing. Typical can be an upper bound for some vendors (meaning all units should outperform it, but it is too expensive to guarantee), or it could be an average for other vendors (meaning some units are better and some are worse). If you see a typical specification, it is good to ask for real test data, and even to verify it yourself using the methods shown above.</span></li>
</ul></ol>The end of the above story was that the customer quickly realized that the sales person from the competition had broken all 10 of the rules above when he said his scope has less noise than ours. Put on equal footing, our noise was a little bit less than theirs, but in truth, both were essentially the same when examined on a whole. In a future blog post, I'll discuss why vertical noise is such a red herring when examining two oscilloscopes. But for now, you are armed with new knowledge to use the next time somebody says, "Hey, want to see my super low noise real-time oscilloscope?"Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com0tag:blogger.com,1999:blog-1565037239206599239.post-83127102885773205672011-06-07T19:41:00.000-04:002011-06-07T19:41:18.117-04:00Tektronix Magician at IMS2011 in Baltimore, MD<iframe width="425" height="344" src="http://www.youtube.com/embed/7P9XABwX4Ro?fs=1" frameborder="0" allowfullscreen=""></iframe>Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com0tag:blogger.com,1999:blog-1565037239206599239.post-77928685600490491562011-05-06T09:52:00.001-04:002011-05-06T09:54:05.781-04:00FFT Speed on Tektronix MSO4104BA customer asked me how fast the FFT appeared on the Tektronix MSO4104B. For thos who are unfamiliar with it, an FFT is a mathematical function that allows you to see the frequency content in a signal that is digitized in the time domain. By doing an FFT, you can use your oscilloscope like a spectrum analyzer. Most benchtop scopes have an FFT system, but on ones that do not run Windows, the FFT is often limited to only 1000 points. The problem is that with only 1000 points, your resolution bandwidth (or frequency visibility) is extremely limited.<br />
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The Tektronix MSO4104B can do very quick and lively FFT analysis up to 100k point records, and can actually crunch a 1M point record into an FFT as shown in the video below.<br />
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<iframe allowfullscreen="" frameborder="0" height="295" src="http://www.youtube.com/embed/2gJ77GS_IUc?fs=1" width="480"></iframe>Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com0tag:blogger.com,1999:blog-1565037239206599239.post-90307728076241617432011-04-14T03:01:00.002-04:002011-08-25T20:15:30.454-04:00Superspeed USB 3.0 Plugfest in Portland, Oregon<div class="separator" style="clear: both; text-align: center;"></div>This week I attended the Superspeed USB 3.0 PlugFest at the Embassy Suites in Downtown Portland, Oregon. Tektronix is headquartered in Beaverton (not far away), so it was a chance to visit the factory and also to learn more about the latest high speed computer serial bus.<br />
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<div class="separator" style="clear: both; text-align: center;"></div><div class="separator" style="clear: both; text-align: center;"></div><div class="separator" style="clear: both; text-align: center;"><a href="http://embassysuites1.hilton.com/ts/en_US/hotels/content/PDXPSES/media/images/photo_gallery/PDXPSES_Embassy_Suites_Portland-Downtown_gallery_accom_ext1_large.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="212" src="http://embassysuites1.hilton.com/ts/en_US/hotels/content/PDXPSES/media/images/photo_gallery/PDXPSES_Embassy_Suites_Portland-Downtown_gallery_accom_ext1_large.jpg" width="320" /></a><a href="http://embassysuites1.hilton.com/ts/en_US/hotels/content/PDXPSES/media/images/photo_gallery/PDXPSES_Embassy_Suites_Portland-Downtown_gallery_accom_lobbyarea_large_5.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="240" src="http://embassysuites1.hilton.com/ts/en_US/hotels/content/PDXPSES/media/images/photo_gallery/PDXPSES_Embassy_Suites_Portland-Downtown_gallery_accom_lobbyarea_large_5.jpg" width="320" /></a></div><br />
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So what is SuperSpeed USB? <br />
<a name='more'></a>Otherwise known as USB3.0, it is the next generation of USB standard. The standard connector housing looks identical to USB2.0, but there the similarities end. The connector actually has both connections for USB2.0 and USB3.0 inside of it. The standard requires that USB2.0 devices work in USB3.0 ports and vice-versa, so each connector is designed to work at the lower speeds if one of the two devices being used does not support SuperSpeed. As you can see in the image below, if you remove the 5 thinner wires in the back of the connector, you have a USB2.0 connector.<br />
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USB2.0, otherwise known as High Speed USB, runs at 480MT/s, does not use spread spectrum clocking, and has 4 wires. There is a ground wire, a voltage wire (VBUS), and a single differential signal. Differential means that the 2 wires carry 1 signal, but the differential pair enables it to be more immune to noise and to travel further without degrading in signal quality. The single differential pair carries both transmit and receive, so the link is bidirectional. The bus is DC coupled and is NRZ encoded.<br />
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Behind the USB2.0 conductors, you see 5 pins for USB3.0 (labeled below in red). USB3.0 runs at 5GT/s, or 10x the speed of 2.0, uses spread spectrum clocking, and has 5 wires. There is a ground pin, and then 4 signal wires. There is a differential path for transmit and a differential path for receive. In other words, there are 2 unidirectional signal paths instead of 1 bidirectional signal path. The bus itself is AC coupled and is 8b/10b encoded. 8b/10b encoding is a technique used to make sure no string of 0's or 1's is so long that receive clock recovery circuits lose lock. There are always transitions on the line that ensure the receiver tracks the transmitter properly. Also, the spread spectrum clocking ensures that the 2.5GHz fundamental clock used for data lines does not radiate too much and interfere with other devices. 2.5GHz is a particularly dangerous signal to radiate because WiFi, Bluetooth, and numerous other devices operate there and nobody wants an unintentionally radiated signal from a non-wireless system to interfere with wireless communication.<br />
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<div class="separator" style="clear: both; text-align: center;"></div><div class="separator" style="clear: both; text-align: center;"><a href="http://4.bp.blogspot.com/-ZQcdm-97JgA/TaaOr7d0ksI/AAAAAAAAAW0/qz2R31keGL8/s1600/usb3plug.JPG" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://4.bp.blogspot.com/-ZQcdm-97JgA/TaaOr7d0ksI/AAAAAAAAAW0/qz2R31keGL8/s1600/usb3plug.JPG" /></a></div><br />
Plugfests are cool events. Everybody brings their device to test and make sure it meets the specification. Why is it important to meet the specification? To be allowed to use the USB symbol, a device must meet certain performance requirements. Tektronix focuses on the electrical physical layer, making sure the edges are the right speed, pulses the right amplitude, and the signals the right timing. Others at the PlugFest focus on interoperability, or the actual communication from the device itself. Everybody plugs their device into various hosts and makes sure that the two can talk (hence, plugfest!). A physical layer test makes sure the signals are correct, but does not make sure the device can actually communicate with a host.<br />
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So follow the sign to the USB Plugfest!<br />
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<div class="separator" style="clear: both; text-align: center;"><a href="http://4.bp.blogspot.com/-x_WSepZOJ8Q/TaaGCXYPzSI/AAAAAAAAAVs/7Du9xosgNv0/s1600/IMG_0242.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="239" src="http://4.bp.blogspot.com/-x_WSepZOJ8Q/TaaGCXYPzSI/AAAAAAAAAVs/7Du9xosgNv0/s320/IMG_0242.jpg" width="320" /></a></div><div style="clear: both; text-align: left;"><br />
</div><div style="clear: both; text-align: left;">Any Superspeed USB device must also be compliant with High Speed USB2.0 and Full Speed USB1.1. Here we are running that test with a Tektronix MSO5204 2GHz oscilloscope and a TDSUSBF compliance test fixture. You'll notice in the photos that I show the Tektronix test equipment, but I do not show the devices we are testing. Many electronics companies are bringing devices that have not yet made it to the market, so they are understandably secretive. Remember how upset Apple was when an iPhone was left in a bar? One company had us test their device while it was wrapped in tape and all we could see was a connector sticking out of the side for us to test. So the photos below will show our test setups, but no devices under test other than things we brought ourselves to test our own setups.</div><div class="separator" style="clear: both; text-align: left;"><br />
</div><div class="separator" style="clear: both; text-align: center;"><a href="http://2.bp.blogspot.com/-VEdUlhsC1C8/TaaK5SrSrgI/AAAAAAAAAWU/vtN-MOT7axQ/s1600/IMG_0270.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="298" src="http://2.bp.blogspot.com/-VEdUlhsC1C8/TaaK5SrSrgI/AAAAAAAAAWU/vtN-MOT7axQ/s400/IMG_0270.jpg" width="400" /></a></div><div class="separator" style="clear: both; text-align: center;"><br />
</div><div class="separator" style="clear: both; text-align: left;">The above setup uses the fixture below to test a device to the Full Speed and High Speed (1.1 and 2.0) standards. Each connector is used for a different test, and the probes connect to the oscilloscope so a special test software called TDSUSB2 can analyze the waveforms. To get the USB device tested, software is run on a separate PC that puts the device into various test modes. Essentially, you hook the fixture itself up to a laptop, command the device under test (DUT) to send out a test packet, and then you turn a switch so the oscilloscope can measure the waveform.</div><div class="separator" style="clear: both; text-align: center;"><br />
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<tr><td style="text-align: center;"><a href="http://2.bp.blogspot.com/-sydJIluCASs/TaaTUje75CI/AAAAAAAAAW8/5aMtqAGwWrs/s1600/usb2eye.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="240" src="http://2.bp.blogspot.com/-sydJIluCASs/TaaTUje75CI/AAAAAAAAAW8/5aMtqAGwWrs/s320/usb2eye.png" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">TDSUSBF Compliance Fixture for Full Speed USB1.1 and High Speed USB2.0</td></tr>
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</div><div>Below is the setup in the USB "gold suite" where official testing is being performed for USB2.0 compliance. The one test not shown in the Tektronix MSO5204 setup above is the jitter tolerance test. Below you can see a Tektronix DPO7254 oscilloscope with a Tektronix AWG5014C below it. The AWG is a signal generator that is playing a simple disruptive pattern into a device under test. On the screen you can see a low level packet followed by a high level packet. Remember that USB2.0 is bi-directional, so both transmit and receive are on the same trace. If the jitter is turned too far up, the transmit packet disappears from the scope display. Thus, a device can be tested for its tolerance of jitter. The rest of the gold suite test on the DPO7254 is identical to what we did above with the MSO5204.<br />
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<div class="separator" style="clear: both; text-align: center;"><a href="http://4.bp.blogspot.com/-vwfuKdFu2-I/TaaGDNiq-YI/AAAAAAAAAV4/PTR97TjOj94/s1600/IMG_0268.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="http://4.bp.blogspot.com/-vwfuKdFu2-I/TaaGDNiq-YI/AAAAAAAAAV4/PTR97TjOj94/s320/IMG_0268.jpg" width="239" /></a></div><div class="separator" style="clear: both; text-align: center;"><br />
</div><div class="separator" style="clear: both; text-align: left;">For SuperSpeed USB (USB3.0), the test fixture is remarkably simpler, but the test setup is far more complex. While a 2GHz oscilloscope is sufficient for USB2.0 (along with a lower cost stimulus/jitter source), USB3.0 requires a 12.5GHz oscilloscope. In addition, USB3.0 requires a receiver sensitivity test with a high performance data generator to make sure the device can handle jitter.</div><div class="separator" style="clear: both; text-align: left;"><br />
</div><div class="separator" style="clear: both; text-align: left;">The fixture below shows a port for injection of a signal from the data source (lower right). It goes through a 3 foot long USB cable (a lossy channel) and it launched through the PCB into the device under test connected to the board as well (top of the image). Then, the device retransmits data to the 2 SMA cables attached to the board which take the data to a piece of test equipment (either an oscilloscope or a BERT).</div><div class="separator" style="clear: both; text-align: center;"><br />
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<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/-LixWCVV_kKg/TaaGDJHPTcI/AAAAAAAAAV0/xIok68j-dig/s1600/IMG_0251.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="239" src="http://1.bp.blogspot.com/-LixWCVV_kKg/TaaGDJHPTcI/AAAAAAAAAV0/xIok68j-dig/s320/IMG_0251.jpg" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Official SuperSpeed USB3.0 Compliance Fixture</td></tr>
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</div><div class="separator" style="clear: both; text-align: left;">Tektronix supports 2 different test suites for Superspeed USB. Below you can see an AWG7122C as the data generator with a Tektronix MSO72004C as the oscilloscope. The test is automated by TekExpress, an automated framework that communicates with both the oscilloscope and the AWG to perform the test. The oscilloscope bandwidth is overkill for USB3.0, so TekExpress limits the bandwidth via digital signal processing (DSP) to 12.5 GHz for the test.</div><div class="separator" style="clear: both; text-align: left;"><br />
</div><div class="separator" style="clear: both; text-align: left;">First, transmitter testing is performed. The AWG send a signal to the DUT called an LFPS, or a low frequency pulse that initiates a pattern to come out of the device. This test pattern can be switched by applying the LFPS again. These test patterns are analyzed by the oscilloscope, and TekExpress indicates pass or fail. Transmitter testing requires a real-time oscilloscope to see the signal.</div><div class="separator" style="clear: both; text-align: center;"><br />
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<tr><td style="text-align: center;"><a href="http://2.bp.blogspot.com/-jfGfVX3sBRc/TacTC3_OfPI/AAAAAAAAAXA/Ny61axIAtvg/s1600/awgscopesetup.JPG" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="185" src="http://2.bp.blogspot.com/-jfGfVX3sBRc/TacTC3_OfPI/AAAAAAAAAXA/Ny61axIAtvg/s400/awgscopesetup.JPG" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Setup for Superspeed USB3.0 Test with Tektronix Oscilloscope and AWG</td></tr>
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<tr><td style="text-align: center;"><a href="http://4.bp.blogspot.com/-juKyM7y9VVY/TaaK265qqaI/AAAAAAAAAWE/C3aJUZJKDaE/s1600/IMG_0246.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="320" src="http://4.bp.blogspot.com/-juKyM7y9VVY/TaaK265qqaI/AAAAAAAAAWE/C3aJUZJKDaE/s320/IMG_0246.jpg" width="239" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Photo of Superspeed USB3.0 Test at PlugFest</td></tr>
</tbody></table><div style="clear: both; text-align: left;">Next, TekExpress performs receiver testing. The AWG sends the command into the DUT to put it into loopback mode. In this mode, the DUT literally just replays any data it receives. If it misunderstands a bit, then it replays the wrong bit and a bit error is detected. To run the test, the AWG plays out different data patterns with various amounts of jitter. The setup is fairly simple because an AWG can literally shape its bits, adding pre/deemphasis, loss, or jitter before it sounds out a signal. To detect the error, we use a Tektronix oscilloscope. Tektronix oscilloscopes are the only ones in the world with a built-in bit error rate detector in the trigger system. It must be done by hardware because any digitized data on he oscilloscope screen will have gaps in it, so a continuous bit-error rate test can only be done with dedicated error detection circuitry. The trigger system of the oscilloscope can actually count bit errors in a serial data stream. TekExpress checks the bit errors and makes sure that the DUT can ignore the extra jitter and transmit the right data.</div><div style="clear: both; text-align: left;"><br />
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</div><div class="separator" style="clear: both; text-align: left;"></div><div style="text-align: left;">The other test setup for SuperSpeed USB3.0 uses the Tektronix BERTScope. Either setup can do the compliance test, and there are advantages and disadvantages to each. In this setup, there is no AWG, but rather a Tektronix BSA85C BERTScope that both generates the data and counts the bit errors. An oscilloscope is still used for transmitter test, but the BERTScope is used for the receiver test. The BERTScope takes the place of both the AWG and the error detector on the oscilloscope.</div><div style="text-align: left;"><br />
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<tr><td class="tr-caption" style="text-align: center;">Superspeed USB3.0 Transmitter test on Tektronix Oscilloscope</td></tr>
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<tr><td style="text-align: center;"><a href="http://3.bp.blogspot.com/-BGqu5B2Rrd4/TaaOxxtiD8I/AAAAAAAAAW4/ne-gISBLnBA/s1600/bertscopesetup.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="316" src="http://3.bp.blogspot.com/-BGqu5B2Rrd4/TaaOxxtiD8I/AAAAAAAAAW4/ne-gISBLnBA/s400/bertscopesetup.png" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Tektronix BERTSCope Setup for Superspeed USB3.0 Receiver Test</td></tr>
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<tr><td style="text-align: center;"><a href="http://4.bp.blogspot.com/-TXhYPut3Ykk/TaaK4OFQI4I/AAAAAAAAAWM/yXMKVio4BaQ/s1600/IMG_0255.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="298" src="http://4.bp.blogspot.com/-TXhYPut3Ykk/TaaK4OFQI4I/AAAAAAAAAWM/yXMKVio4BaQ/s400/IMG_0255.jpg" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Photo of Setup at PlugFest</td></tr>
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</div><div style="clear: both; text-align: left;">In the image below, the Tektronix BERTScope BSA85C generates a data pattern into the DPP, or digital pre-emphasis processor (the box on top of the stack). The AWG does not need the DPP since it can shape the bit shape directly, but since the BERTScope is a high speed digital generator, it needs the extra processor to add the pre-emphasis. There is also a Tektronix CR125A clock recovery unit that is used for receiving the data when receiver testing is performed (show sitting directly on top of the BERTScope on bottom).</div><div class="separator" style="clear: both; text-align: center;"><br />
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<div class="separator" style="clear: both; text-align: center;"><a href="http://1.bp.blogspot.com/-3ygwdeVtI60/TaaK1soXCQI/AAAAAAAAAV8/tJbU5zLF4kA/s1600/IMG_0272.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="http://1.bp.blogspot.com/-3ygwdeVtI60/TaaK1soXCQI/AAAAAAAAAV8/tJbU5zLF4kA/s320/IMG_0272.jpg" width="239" /></a></div><div style="clear: both; text-align: left;">On top of the BERTScope is a special instrument switch that can initiate the LFPS signaling required to toggle the DUT and put it into loopback mode. It is shown side-on in the image above, so I took a better photo in the image below. LFPS is tough for a BERTScope to generate since it is low frequency, so this box generates the LFPS and then switches to allow the BERTScope to communicate with its 5GB signal.</div><div style="clear: both; text-align: left;"><br />
</div><div class="separator" style="clear: both; text-align: center;"><a href="http://1.bp.blogspot.com/-XMz_XLjQ0fE/TaaK4cPTTeI/AAAAAAAAAWQ/BXEQVRw6w2Y/s1600/IMG_0262.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="239" src="http://1.bp.blogspot.com/-XMz_XLjQ0fE/TaaK4cPTTeI/AAAAAAAAAWQ/BXEQVRw6w2Y/s320/IMG_0262.jpg" width="320" /></a></div><div class="separator" style="clear: both; text-align: center;"><br />
</div><div style="clear: both; text-align: left;">In this picture, we were debugging a DUT and trying to cycle through various test patterns. Jit Lim from Tektronix (see his blog here <a href="http://www.edn.com/blog/Scope_Guru_on_Signal_Integrity/index.php">Jit Lim - The Scope Guru</a> ) is firing off the LFPS signals.</div><div class="separator" style="clear: both; text-align: center;"><br />
</div><div style="text-align: center;"><a href="http://3.bp.blogspot.com/-DxHbhc1jkJY/TaaK3lvBFjI/AAAAAAAAAWI/F7o28ETv6xM/s1600/IMG_0250.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="http://3.bp.blogspot.com/-DxHbhc1jkJY/TaaK3lvBFjI/AAAAAAAAAWI/F7o28ETv6xM/s320/IMG_0250.jpg" width="239" /></a></div><br />
<div class="separator" style="clear: both; text-align: left;">The other really cool thing that the BERTScope can do is called margin testing. The AWG can play out the proper waveform with the prescribed jitter, but the BERTScope can easily adjust the jitter up until the device breaks and stops properly transmitting. The blue area below shows the required performance, and the vertical Y-Axis is a log scale of jitter as a percentage of the unit-interval (UI). The x-axis is jitter frequency. The BERTScope tests each jitter frequency at increasing levels (shown by the green dots) until the device stops responding (shown by the black dots and the black line. This vendor's device is superb and well exceeds the specification for jitter tolerance.</div><div class="separator" style="clear: both; text-align: left;"><br />
</div><div class="separator" style="clear: both; text-align: center;"></div><div class="separator" style="clear: both; text-align: center;"><a href="http://2.bp.blogspot.com/-d3lGnbR7exo/TaaNRsbxEeI/AAAAAAAAAWw/ohffCZO2aiA/s1600/IMG_0257.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="235" src="http://2.bp.blogspot.com/-d3lGnbR7exo/TaaNRsbxEeI/AAAAAAAAAWw/ohffCZO2aiA/s320/IMG_0257.jpg" width="320" /></a></div><div class="separator" style="clear: both; text-align: center;"><br />
</div><div class="separator" style="clear: both; text-align: left;">Both the AWG/Oscilloscope and BERTScope/Oscilloscope did a nice job of testing the devices, but for speed, ease of use, and margin testing, I definitely preferred working with the Tektronix BSA85C BERTScope Both tools, however, gave excellent results and quick answers, verifying devices met the compliance specification.</div><div class="separator" style="clear: both; text-align: center;"><br />
</div><div class="separator" style="clear: both; text-align: left;">I'm now heading back to Baltimore after a tiring couple of days, but I learned quite a bit about this new technology and hope you did as well!</div><div><div class="separator" style="clear: both; text-align: center;"></div><div class="separator" style="clear: both; text-align: left;"><br />
</div></div></div>Joel Avruninhttp://www.blogger.com/profile/07396200850597065455noreply@blogger.com1