Understanding the Effects of Inputting High Frequency Signals on an Oscilloscope

[reddvoid]

what happens if we input higher frequency signal to oscilloscope than its range. How will It respond. . .for example in oscilloscope range is 12 MHz how will it display a signal having rise time of say some 40 nano seconds ?

 

[yungman]

Usually it just does not displace the full amplitude, the rising and falling edge is slower than the real signal. If the signal is a lot higher in frequency, it might not even display any waveform, but show up as some DC offset.

 

[berkeman]

It can also alias when its sampling frequency is too low for the input signal. That shows up as a nonsense waveform, or it can show up as a low-frequency waveform (which can be very confusing).

 

[yungman]

Yes, I kept thinking about analog scope. For digital scope, you can have beat frequency that you can see low frequency patterns when the input frequency come close to half clock, clock frequency, 2 clock and so on. That's how I test the A to D front end when I was working for LeCroy long time ago. In fact, these are the most important test on how good the ADC is. You see missing codes using these beat frequency technique.

 

[sophiecentaur]

Usually it just does not displace the full amplitude, the rising and falling edge is slower than the real signal. If the signal is a lot higher in frequency, it might not even display any waveform, but show up as some DC offset.

It can also alias when its sampling frequency is too low for the input signal. That shows up as a nonsense waveform, or it can show up as a low-frequency waveform (which can be very confusing).

Yes, I kept thinking about analog scope. For digital scope, you can have beat frequency that you can see low frequency patterns when the input frequency come close to half clock, clock frequency, 2 clock and so on. That's how I test the A to D front end when I was working for LeCroy long time ago. In fact, these are the most important test on how good the ADC is. You see missing codes using these beat frequency technique.

Definitely!
Basically, there are many worse things possible than just a soggy high frequency response and you can miss all sorts of characteristics of a waveform if you take what you see as the literal truth. Scopes, these days try to think for you and you can lose important features if you take the first picture you see as gospel.