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.
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.
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......
Why does attenuation matter so much?
If you look online, there are lots of tutorials to help you pick a probe. There's even an "app for that" 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.
Remember back to my post on the V/div knob. 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.
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.
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.
In the following screenshot taken on a Tektronix MSO4104B, 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.
Also note that I made these measurements using "hi-res" mode and running at 500MS/s. This should provide me about 220MHz and 9 effective bits.
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 Tektronix TPP0502. 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).
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.
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.
|Tektronix P52xx Series|
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 P5202A is 20x or 200x, the P5205A is 50x or 100x, and the P5210A is 100x or 1000x.
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.