I made a bit more work on the interpolation algorithm, I haven't finished yet... but for the time being I will stop for now to work on the interpolation algorithm as I have achieved some good result today.
So the Oscilloscope is still sampling only at 100 MHz! (200 MHz next week end I guess).
Let see some waveform:
1 MHz Sinewave
Not too bad I think... it is still possible to see some issues due to common mode noise that probably goes inside the input ( 1MHz) of the oscilloscope and is displayed, as at moment the oscilloscope is still not isolated from the PC I must leave with it until I isolate it.
1 MHz squarewave
Not too bad I think, considering that the square wave is the function with lots of harmonics .
5 MHz sine wave
not too bad I think
5 MHz square wave
Not too bad considering I'm sampling at 100 MHz!
10 MHz sine wave
10 MHz square wave
Not too bad considering that the input is a 10 MHz square wave and I've got only 10 samples to rebuilds.
I wish to thanks all the person that are following this blog for the excellent suggestions on how to design the interpolation algorithm. When you buy an oscilloscope and you read sinc(x) interpolation. Don't believe it without a good windowing you have overshoot all over the place. in my case I'm using a sin(x)e(-x^2/3) interpolation, the Gaussian works pretty well in reducing the overshoot.
What to do now?
1) improve the SW
2) finish to design the isolator ( the noise should further reduce)
3) sample at 200 MSPS ( so far I haven't done because of a mistake of mine in choosing the clock pin of the FPGA)