Wednesday, March 30, 2011

Oscilloscopes Exposed - The Volts Per Division Knob

Customers commonly ask me these questions:
  • Why do my measurements change as I adjust the volts/division knob (also called scale)?
  • Why does noise increase at larger volts/division?
  • Why might it be a mistake to compare 2 oscilloscopes from different manufacturers at the same volts per division setting? (this is a big issue as you will see below)

Friday, March 25, 2011

Book Review: Cognitive Radio Communications and Networks Principles and Practice

I wanted to take a break from my normal blogging to discuss a new book I just read called "Cognitive Radio Communications and Networks: Principles and Practice".  In addition to being a scope guy, I also support the Tektronix Real Time Spectrum Analyzer, a very popular tool for software defined and now cognitive radio development.  The RSA allows you to visualize live RF in real-time (and then trigger and capture it), so it is a great tool for software defined radio development.  As a result, I am often brought to customers and regulatory bodies that are investigating the technology behind cognitive radio and need our equipment to verify operation.  I got this book to improve my own knowledge in the field.

Tuesday, March 22, 2011

Tektronix Logic Analyzer in the Movie FaceOff

No sooner had I made the comment in my first blog post about Tektronix being in all the movies than people started e-mailing me with the names of movies in which they remember seeing a Tektronix oscilloscope.  Most of the time it is in the background and not the focus, but one person told me of a Tektronix Logic Analyzer being the star of a movie with Nicholas Cage.  Naturally I had to go straight to NetFlix to get the movie and find out.

Remember Face/Off from 1997 with Nicholas Cage and John Travolta?  Cage plays a bad guy who plants a bomb, so naturally when he is critically wounded, Travolta must undergo surgery to take Cage's face and find out from Cage's cohorts where the bomb is located.  After the surgery, the doctor wants to test the new voice altering chip he implanted, so naturally he turns to the Tektronix Logic Analyzer, a TLA715.  In 1997, this would have been a fairly new piece of equipment so it's surprising they were using it.

I've never seen this particular application running on a logic analyzer---perhaps it was written by one of our Applications Engineers? Either way, it's cool and I want it!

(for those who don't know, a logic analyzer is a way to acquire digital 1's and 0's, and would not be used for audio or voice analysis as is shown here):

I made a screen capture of the whole scene with my iPhone..... check it out!  Lots of up-close shots of the Logic Analyzer.

And in case you're curious, the modern equivalent would be the TLA7012, pictured below on the right, next to his big brother, the TLA7016.  Just also happens to be the fastest logic analyzer out there,so check it out!

Real-Time Oscilloscopes Versus Sampling Expanded

I got such great feedback about the last post on the differences between real-time and sampling oscilloscopes that I thought I would expand those thoughts a bit for this post.

Real Time OscilloscopeSampling Oscilloscope
TriggeringNone NeededNeeds External Trigger
Trigger TypesPinPoint, Internal, ExternalExternal Edge or Clock Recovery
Max Sample Rate100GS/s*200kS/s
Single Shot?YesNo, Must Be Repetitive
Catch Glitches?YesNo
Record LengthUp to 500M points16000 points
Minimum Time Resolution10ps*10fs
Timing AccuracyGoodExcellent
Low NoiseGoodExcellent
Veritcal Resolution8-bits16-bits
Capability for TDR?NoYes
Optical Portfolio?LimitedExtensive
Top Bandwidth?33GHz70+GHz
Price for a 20GHz solution>$150k<$50k
Tektronix MSO72004C and Tektronix DSA8300

* Real-Time Oscilloscope in Equivalent Time Mode (operating similar to a sampling oscilloscope) can reach 10TS/s or 200fs time resolution, but same limitations of sampling scope apply (signal must be repeated)

Monday, March 21, 2011

Difference Between Analog, Digital, and Sampling Oscilloscopes

In my last post, I used the word “oscilloscope”, but it’s entirely possible that I should have qualified it to say “real-time oscilloscope”.  What is a real-time oscilloscope and why do we call it real-time?  How is it different from an equivalent time or sampling oscilloscope?

First we must note that almost every oscilloscope on the market today is a digital oscilloscope.  Old-time engineers still remember and love their analog oscilloscopes.  I won’t spend too much time explaining the operation, but an analog oscilloscope is literally a Cathode Ray Tube (CRT like your old TV) where an electron beam sweeps across the display while an external signal deflects the signal up and down.  The speed of the sweep across the display is what we think of as “timebase” today, and the level of vertical deflection is like our “volts per division” today.

Friday, March 18, 2011

All about numbers BIG and small

I am writing up another post right now on analog scopes vs digital scopes vs sampling scopes, and I realized that many abbreviations I use may not be familiar.  So here are some of the most commonly used big numbers in digital oscilloscopes.
  • 1 S/s – this is 1 sample every 1 second, or very slow
  • 1kS/s – 1 kilo-sample per second, 1000 = 1e3
  • 1MS/s – 1 mega-sample per second, 1,000,000 = 1e6
  • 1GS/s – 1 giga-sample per second, 1,000,000,000 = 1e9
  • 1TS/s – 1 tera-sample per second, 1,000,000,000,000 = 1e12
And of course, memory is specified normally in terms of samples.  So if somebody tells you their scope has 10M of memory, it’s not mega-bytes but megasamples.  This simplifies the math.  A scope with 10M of memory running at 1MS/s can store 10 seconds of data continuously!
Now numbers also get small:
  • 1ms – 1 millisecond – at 1kS/s, sample pts are .001 seconds apart = 1e-3
  • 1us – 1 microsecond – at 1MS/s, .000001 seconds apart = 1e-6
    • This is really a greek mu, a funny looking u, but it’s easier to type u
  • 1ns – 1 nanosecond – at 1GS/s, .00000001 seconds apart = 1e-9
  • 1ps – 1 picosecond – at 1TS/s, .0000000001 seconds apart = 1e-12
  • 1fs – 1 femtosecond = 1e-15
  • 1as – 1 attosecond = 1e-18
Now why do I mention femtoseconds and attoseconds?  First, in equivalent time mode, samples can get 100fs apart on a realtime oscilloscope and 10fs apart on a sample oscilloscope.  Second, even though real-time sampling at its fastest puts sample points 10ps apart, jitter software performs calculations that can result in random jitter numbers measured in femtoseconds.  I’ve seen random of 140femto-seconds on a Tektronix DPO72004C.

And why do I mention atto-seconds?  It turns out that in some calculations (such as perfectly gaussian low-noise jitter with the right clock recovery), the mean “time interval error” is measured in atto-seconds, or “as”.  I’m not sure how meaningful an atto-second really is, but it certainly looks better than rounding to zero!

So that’s your brief intro to the world of abbreviations in scopes.  Now I can post with a clear conscience!

Sunday, March 13, 2011

The World of Oscilloscopes in 2011

Welcome to the first post of Joel Avrunin’s electronics blog.  Here I am going to talk about anything of interest to me technologically, with a focus on my primary interest, test and measurement.

Joel Probing a High Speed Serial Signal with a Tektronix BERTScope
In today’s blog entry, we will start a series of conversations related to the most fundamental tool of an engineer, the oscilloscope.  I always tell people that an oscilloscope is the worst tool to do any job but the best tool to do every job.  Just about every measurement you can make with an oscilloscope can likely be done better with a purpose-built test setup, but the ubiquitous oscilloscope can do it all.

Most engineers don’t remember their first network analyzer, but they all remember their first oscilloscope.  A first oscilloscope to an engineer is a rite of passage.  Most people think of their childhood and remember their first kiss, their first beer, their first car.  Engineers remember their first oscilloscope, and it always holds a special place in their heart (not to say we neglect the other firsts….).

Please add to the comments of this post and tell me about your first oscilloscope – even if it’s not Tektronix, I’d love to hear the story!

At its simplest, an oscilloscope is a tool that lets you visualize electrical signals, displaying the signal on a graph with voltage on the vertical Y-axis and time on the horizontal X-axis.

In the world of oscilloscopes, one company owns more than half the market and is about the oldest name in the industry, Tektronix.  That is my company, and though my blog will be technologically focused (and not one big ad for Tektronix), I will often use Tektronix oscilloscopes for illustration due to the fact that they are the industry standard and I do work for Tektronix.  Tektronix consistently leads the industry in bandwidth, noise performance, effective number of bits, and sampling rate.  It is a technology driven company and that shows in the many products that have come out over the years.  Other companies may occasionally beat Tektronix to a performance mile mark, but over the years, Tektronix stays consistently ahead as the market leader.  In my humble opinion, Tektronix also understands how an engineer uses an oscilloscope and still has the best user interface in the industry.

Many people think Tektronix invented the oscilloscope, but that distinction belongs to Karl Braun who invented the first oscilloscope in 1897.  Tektronix’s founders made the oscilloscope useful for the exploding digital world that was to occur following WWII through the invention of the “trigger”.  Prior to Howard Vollum and Jack Murdock founding Tektronix, oscilloscopes could display a waveform but not trigger on it.  All of that changed in 1947 with the introduction of the Tektronix 511 – the first triggered oscilloscope.  We’ll talk about why a trigger is important in later blog entries.

First Trigger Oscilloscope - Tektronix 511

One thing I love about working for Tektronix is that you can find circuits in textbooks developed by Tektronix engineers, and 65 years of patents on technologies we take for granted today.  Tektronix innovated many display techniques for CRT’s and even invented the technology behind Plasma TV.  And of course, whenever Hollywood wants a laboratory to look high-tech, they always have a Tektronix scope on the rack.

When John Locke sat at the computer in “Lost” typing those digits again and again, two Tektronix 502A Dual Beam Oscilloscopes sat behind him, doing something really important (although they never explained what!).  These dual beam oscilloscopes were immensely popular, and it is cool to see them immortalized in one of the best TV shows in recent times.

When I show a customer a new Tektronix MSO4104B, I am often treated to a story about their first Tektronix scope and how it changed their life.  Did I mention it is great to work for a legendary company?

Another major player in the world of test and measurement is Agilent (formerly known as Hewlett-Packard or HP).  Mostly known for their RF portfolio (spectrum analyzers and network analyzers), they have been in the oscilloscope market for many years.  They are still a distant number 2 player to Tektronix, but have made many improvements to their portfolio recently.  Having lagged Tektronix in bandwidth for the past 5 years, they finally introduced a higher bandwidth real-time scope last year to claim bandwidth leadership (for now….).  Recently, they introduced their own line of bench scopes, after selling OEM-labeled Rigol scopes for many years.  In the world of oscilloscopes, they are a solid #2.

Picking the number 3 company is difficult because it likely depends on how you slice the market.  In the world of performance oscilloscopes (anything above 4GHz), the only other name is LeCroy.  I used to be a design engineer at LeCroy, so I have a special place in my heart for their equipment.  What did I tell you about engineers and their first oscilloscopes?

Walter LeCroy invented the Digital Storage Oscilloscope (DSO) in 1985, based largely on work done with digitizers at CERN in Switzerland.  They have had their good years and bad years.  While the digitizer quality is arguably not up to the level of Tektronix or Agilent, these oscilloscopes tend to be very popular with physicists due to the embedded analysis capabilities.  One look at a LeCroy scope user-interface and you’ll know it was an oscilloscope designed by physicists.  They also have innovative industrial design that looks “cool”.  They are also popular in Europe, likely because they still do have a design center in Switzerland and the name sounds French, but the company is based in New York.  Their base digitizer technology is not as fast as Tektronix or Agilent, but they do some interesting things with frequency mixing on their higher bandwidth scopes that currently give them the highest real-time banner specification in the industry.  They used to have a relationship with Iwatsu for the lower bandwidth/bench scopes, but I believe that recently ended and Iwatsu is marketing their own oscilloscopes, mostly in Japan.

I don’t have the market data to separate the next oscilloscope makers.  The next biggest players in the “mid-performance” market (beyond Tektronix, Agilent, and LeCroy) are likely Yokogawa and now Rohde.  Yokogawa is extremely well known in Japan, and they have some interesting products.  The user interfaces often don’t play very well with Western engineers, but I see them in the United States in automotive and power applications.  The automobile manufacturers love their lower bandwidth scopes with extremely deep memories typically only seen on higher bandwidth scopes.  Also, Yokogawa makes scopes with more than 4 analog channels.

Rohde is a pretty exciting newcomer to the oscilloscope world.  Rohde is a premier RF company based in Germany, and for years the Tektronix sales force established their name in the United States, primarily competing with the Agilent RF portfolio.  Their relationship ended several years ago, and Rohde really only participated in the low end of the oscilloscope market through their Hameg oscilloscopes.  Last year, Rohde made headlines with a new oscilloscope family going up to 2GHz.  Key features they advertise are the digital trigger, fast update rate, rapid FFT processing (spectral analysis), and adjustable display grids.  They are still a small player, but that could change in the future.

Beyond Tektronix, Agilent, LeCroy, Rohde, Yokogawa, and Iwatsu, there are a ton of smaller players when you talk “bench” scopes, or oscilloscopes less than 500MHz.  To throw out some names, there are Fluke, Siglent, Tekway, ABI, B&K Precision, ExTech, Digimess (Grundig), GW Instek, Pico Technology, Tenma (Newark), and names you will only see on eBay like Gould, Nicolet, and Leader.  National Instruments makes digitizer cards, some with Tektronix technology, but when I talk about oscilloscopes, I mean something with knobs and a screen.

That’s the world of oscilloscopes, and I hope to blog at least once a week on a topic I find interesting, or one suggested to me by a customer or a reader.  In the future, I plan to discuss what to look for in a new scope (bandwidth, sample rate, memory, ENOB, update rate, application support, probing, etc) and what to look for in a low-cost used oscilloscope.  I hope to keep my blog applicable to a home hobbyist with a $500 oscilloscope and to a power-user designing PCI-Express Gen 3 interfaces, Wideband Satellite Communication, or Radar systems.  I welcome comments as well, but please keep discussions civil.

As I said above, please fill in the comments of this article with some memories of your favorite first oscilloscope.