Hi,
I am going to speculate that more fundamental issue is the ratio between bandwidth and the sampling rate.
The Nyquist Sampling theorem tell us the bandwidth is half the sampling frequency. Then you get folding and aliasing.
There is some modulation caused by the sampling frequency.
If you look at V(sample) waveform, You have to guess at the input waveform from the sampled data.
There isn't enough data to create the waveform with any degree of confidence.
A 100Mhz scope with 500Msps isn't enough data points.
Sorry, but that is the wrong conclusion. Nyquist says that all the information from a signal is there up the fs/2. It is just that your brain can't make a signal from the dots. But that is a problem in your brain and not in the number of samples. In order to help your brain DSOs have sin x/x reconstruction which connects the dots in a mathematically correct way to show you the signal. Because there needs to be some headroom for an anti aliasing filter most DSOs have a maximum bandwidth of fs/2.5 .
Sure, there are ways to reconstruct a sinewave from 2.5 samples per period.
But I believe the signal has to be continuous and have no frequency content beyond fs/2
If you reconstruct a sinewave from a small number of data points, you are making the assumption that the signal was sinewave in the first place.
Regards,
Jay_Diddy_B
You are absolutely correct. If you are looking at 100 MHz squarewave signal on 100 MHz scope you should get 100 MHz sinewave on screen.
Because your scope must not show 300 MHz and 500 MHZ and 700 MHz at any amplitude to be visible on screen. Built in filtering must take care of that that neither of those frequencies and sharp changes in signal ever reach A/D converter.
If you need to look at 100 MHz squarewave, you need scope with 1GHz bandwith, to see at least first 9 harmonics..
Now, let's say we phase shifted the 100MHz square wave by 1 degree. What sample rate would it take to be able to accurately portray the difference between the original square wave and the one that we just phase shifted by one degree?
You need 1GHz scope to see 9th harmonics of 100MHz squarewave. That would make decent approximation of squarewave, but still bumpy and not perfect. Sampling at 5 GS/s, at 200 ps intervals.
1% of phase shift according to what ? Scope have triggering system to synchronise to signal. Trigger point is synch point.
And that would mean shifting sample point 100ps. What do you expect to see? It won't make a squat of the difference on your 3.5 ns edges on your 100MHz squarewave. Because that is what edge will look like on 100 MHz scope even if you put 40ps edge in it.
On a 1GHZ scope it will have 300-400ps edge.
Every time this kind of topic pops up, I'm sad that in schools they don't plot math functions and solve graphical solutions by hand on millimeter paper and pencil anymore. That was great way to get good feeling for
sense of scale.
Get piece of paper (or CAD program that works in scale if you wish) and plot 100MHz squarewave from horizontal points of 200 ps. And then from 400 ps points. That is respectively 5 and 2.5 GS/s. Than step away from plot and squint at it so it is roughly the size of scope screen form your perspective...
Writing this I think I start to understand where confusion is coming from. People who say that you need at least 10-20 points or more per period to reconstruct arbitrary shape signal are correct. But that is because to look at 100MHz sinewave you need 100 MHz scope (with more than 250 MHz sampling). In order to look at some arbitrary shaped signal repeating at 100 MHz, if you were to look that signal on SA, you would see, say, harmonics going to 2 GHz or more. So to look at that signal you need 2 GHz scope, although signal is periodic with repetition rate of 100 MHz, because that is signal with 2 GHz bandwidth.
Square wave consists of negative harmonics on quite simple formula, 3rd harmonic with 1/3 of amplitude, 5th harmonic with 1/5 of amplitude etc.. To infinity. Of course, very soon you don't need to go further, so in practice on scopes for visual representation, after you pass 11th or 13th harmonic you don't need to go further.. So that is your formula,. Repetition rate of your squarewave multiplied by 10-15. That is your scope bandwidth needed. And that at least 2.5 more will give sampling frequency.
So 100MHz squarewave, 1GHz scope, 2.5 GS/s sampling.
If you could afford 2GHz scope with 10 GS/s that would be better.