Oscilloscope bandwidth is a tricky thing. A 100 MHz scope will have a defined attenuation (70%) of a 100 MHz sine wave. That’s not really the whole picture, though, because we aren’t always measuring sine waves. A 100 MHz square wave, for example, will have sine wave components at 100 MHz, 300 MHz, and the other odd harmonics. However, it isn’t that a 100 MHz scope won’t show you something at a higher frequency — it just doesn’t get the y-axis right. [Daniel Bogdanoff] from Keysight decided to think outside of the box and made a video about using scopes beyond their bandwidth specification. You can see that video, below.
[Daniel] calls this a “spec hacks” but they aren’t really hacks to the scope. They are just methods that don’t care about the scope’s rated bandwidth. In this particular spec hack, he shows how the frequency counter using a 70 MHz scope’s trigger circuit can actually read up to 410 MHz. A 100 MHz scope was able to read almost 530 MHz.
It is interesting that the frequency counter works from the scope’s trigger circuitry which has its own signal path and isn’t going through the analog to digital converter. Of course, your scope might also have a frequency measurement function that reads the trace on the screen. That would probably work too, because you don’t really care how much the signal is attenuated just to read the frequency. Regardless, this is a neat trick to have in your bag.
Honestly, we are surprised the scope makers don’t characterize and rate this as part of the spec. We are pretty sure garden variety probes on a 70 MHz scope aren’t behaving as promised at 400 MHz, so maybe that’s part of the reason why they don’t.
If you have some spare cash, you could just get a 110 GHz oscilloscope, which ought to be enough for anyone. Of course, there’s more to a scope than just its bandwidth, but if you want to learn more about testing bandwidths, we’ve done that on some scopes both new and old.