KeysightOscilloscopes

What is oscilloscope system bandwidth and how do I find the bandwidth of the scope + probe?

Blog Post created by KeysightOscilloscopes Employee on Sep 1, 2016

By Taku Furuta

 

“I am using a 100 MHz oscilloscope with an included 100 MHz passive probe, I am supposed to be able to measure a 90 MHz sine wave, right?  Is the scope or probe broken?”

I hear this sort of question popping up from time to time, understandably since most oscilloscope datasheets do not discuss the “system bandwidth” or your effective bandwidth when a scope is used with a specific probe.

Both an oscilloscope and a probe have bandwidth specifications, the frequency value where the amplitude of input signal attenuates by 3 dB.  So, if your scope’s datasheet specifies its bandwidth at “100 MHz”, you are guaranteed to measure at least ~70% of your signal amplitude at its bandwidth frequency.  The same can be said for your probe as well.  The tricky part is, however, your oscilloscope + probe bandwidth, or your “system bandwidth”, may not be 100 MHz when you use them together. So, what is the system bandwidth in this case?

Before knowing your system bandwidth, you need to know the front end filter response of your oscilloscope.  You may or may not find this info in the datasheet, so call your scope’s support line if it is not stated.  If you don’t want to call/write the support line, I’ll provide you a quick tip to figure this out by just looking at the calculated rise time specifications in datasheets at the end of this blog.  However, it is a good rule of thumb to think the filter is a “Gaussian” type if the bandwidth of your scope is below 1 GHz.  For oscilloscopes with 1 GHz or more bandwidth, it could have a filter type called a “Brickwall”.

In the case of the Gaussian filter, which is the traditional front end filter type used for decades in both analog and digital storage oscilloscopes, the scope and probe’s system bandwidth is calculated using the below formula.

Let’s apply the above example to this formula.  Since your scope’s and probe’s bandwidth are 100 MHz each, your system bandwidth will be 70.7 MHz.  In other words, your signal’s amplitude is attenuated by 3 dB at 70.7 MHz.  Obviously, you will not see full amplitude of a 90 MHz sine wave!

In reality, most of oscilloscope manufacturers add some margin to the bandwidth specifications of both scopes and probes.  So, if you see the specification says “100 MHz”, it most likely has some additional bandwidth, like 110 or 120 MHz.

Now, say if you have a “Brickwall (or maximum flatness)” type filter response oscilloscope and probe instead.  It is extremely rare to see the Brickwall filter on a 100 MHz scope, but for this example say you did.  In such case, unfortunately, the former “square root of sum of squares” formula cannot be used.  In this case, the system bandwidth formula will be:

System Bandwidth = min {scope bandwidth, probe bandwidth}

So, if I apply the original example to this formula, your system bandwidth is now at 100 MHz, therefore, you should see nearly full amplitude of your 90 MHz sine wave.

I am not sure why this simple formula has disappeared from most oscilloscopes’ datasheets.  Perhaps there is more than sufficient bandwidth in most oscilloscopes today where engineers do not need to operate them at their upper limits.  Perhaps this is already taught in school.  Nevertheless, this is a quite useful tip to know, especially if you are seeing unexpected measurement results.

BTW, here is a quick and dirty way to determine if your scope has the “Gaussian” or “Brickwall” type response filter.  First, find your scope’s calculated rise time info.  The below is an example from Keysight InfiniiVision 4000 X-Series oscilloscope.

Now, divide “0.35” the calculated rise time value.  In the case of the 200 MHz oscilloscope (4022A), it will be

0.35 / 1.75 ns = 200 MHz

So, you verified the coefficient it was used to calculate the rise time was “0.35”.  0.35 is the coefficient value for a “Gaussian” response filter, so you know this 200 MHz oscilloscope has a Gaussian filter front end.  On the other hand, if you apply the same formula to 1 GHz oscilloscope (4104A),

0.35 / 450 ps = 778 MHz

The value was 778 MHz and not 1 GHz.  Well, you now know the coefficient used for this oscilloscope was not “0.35”, but was “0.45” (0.45 / 450 ps = 1 GHz).  When the coefficient value is larger than 0.35 such as 0.4, 0.45 or even 0.5, it indicates the scope’s front end has a filter response closer to the Brickwall filter.

Hope this small tip helps you to understand the scopes better.  See you all in the next blog!

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