As electrical engineers there are certain rules that we live and die by each day. Probably, the most common of these is Ohm’s law. No matter what we are doing it always seems to come back to V = I*R, doesn’t it? That silly little equation we learned way back in our youth, maybe in Engineering 101 or our first Physics class, lays the foundation for everything in our field. But what about the other equations and rules that we worship and obey like a zombie survival guide during the apocalypse? Likely, names like Nyquist and Kirchhoff come to mind. And as we delve further into our specific fields, the more specific and sometimes diversified these rules become – after all, there are many different zombies out there and each needs its respective weapon. Sometimes, that weapon is an oscilloscope.
So what rules or guidelines are you thinking about when you are planning to purchase a scope? Probably, you’ve thought again of things like Nyquist in terms of sample rate. You’re thinking about memory depth and waveform update rate. And the noobs, less likely to survive a zombie attack, might be thinking, “Give me the highest possible bandwidth!” But you, seasoned veteran of the apocalypse, obviously know that more bandwidth is not necessarily better.
A good rule of thumb for selecting the bandwidth of your dreamy new oscilloscope is to choose a bandwidth that is 3 times the fastest frequency content in the signal you are looking to analyze. This rule of thumb is for analog signals. If you are on the digital side, then your rule of thumb is 5 times the clock rate of your digital signals. There is a great blog post below (What is oscilloscope system bandwidth and how do I find the bandwidth of the scope + probe) and app note (Evaluating Oscilloscope Bandwidths for Your Application) that go into more details on this if you want to get into the nitty gritty of the nerdy and work out some equations.
But in general, the 3x for analog and 5x for digital BW rule of thumb guarantees that you will have enough bandwidth to properly observe your waveform without taking in too much high frequency content, which will show itself as noise on your desired measurement. Noise is Gaussian, so a higher bandwidth scope sees higher frequency noise.
But what if, you have been bitten by the Maximum Bandwidth Zombie? You came down with the fever and couldn’t turn your mind away from that crazy high bandwidth scope, even though most of your applications are really only operating around 2 MHz or so. Or maybe you simply purchased a scope for a higher bandwidth application than what you need in this very moment. Perhaps, in most cases you are after Runner zombies so you purchase a high bandwidth scope, but occasionally you have to deal with the standard Walkers. Don’t worry, your oscilloscope is a many facetted weapon. This is probably a situation in which you will want to apply bandwidth limiting.
So you turn on bandwidth limiting and suddenly you’ve gone from having a noisy signal and may be experiencing ghosting (a situation in which you’re seeing an additional waveform capture on the screen) to having a nice clean waveform capture.
Here’s an example. Below is a screenshot from a Keysight MSO-X 3104T. This scope has a bandwidth of 1 GHz. On channel 1, I’ve input a 1 MHz sine wave and, for demonstration purposes, mixed it with noise from an 80 MHz function generator. Because I’m using a 1 GHz scope, I’m observing my desired 1 MHz sine wave distorted with the noise from the function generator and any noise in the environment that the scope or probe configuration might be picking up. You’ll also observe the extra, faint signal on screen. This is the ghosting effect I referred to earlier. This is happening because the scope is sometimes triggering on what appears to be the falling edge of the desired signal but actually a rising edge in the noise. This is not pretty measurement, am I right?
Figure 1 – 1 MHz signal with noise
Now, I select the Channel 1 menu and I turn on BW Limit. Bandwidth limiting can be applied to each channel separately. The Bandwidth Limit feature on this scope reduces the maximum bandwidth to 20 MHz.
With the bandwidth limit turned on, the high frequency noise content has been filtered out, and the desired crisp waveform is what remains. See figure 2. One zombie down.
Figure 2 – 1 MHz signal with noise + BW Limit turned ON
Make sure to check out the bandwidth limiting capabilities of your oscilloscope. Keysight has a wide range of options depending on the scope you are using. For example, the InfiniiVision 6000 X-Series oscilloscope lets you select a 200 MHz BW Limit in addition to the 20 MHz BW Limit option shown above on the 3000T X-Series. The Infiniium scopes offer even more possibilities. For example, the MSO-S804A is an 8 GHz scope and allows you to emulate a 6 GHz, 4GHz, 2.5 GHz, 2 GHz, 1 GHz, and even a 500 MHz scope.
As I said before, Keysight is here to help you slay all forms of zombies.