Hello.. is there a way tht we can make a pc oscilloscope which have atleast 20MHz of bandwidth??? ,, no matter what port to interface it with... can anybody provide me with circuit diag?
Bear in mind you'll need to sample at at least 40MHz but to be of any use you really need to go even higher. Sampling at 40MHz will cause you to loose most of the phase information at 20MHz. For example, you have two 20MHz signals 90° out of phase, they appear to be in phase because the deplay between the two signals is 1/80M 1/40M. I'd recommend sampling at 100MHz or even higher.
Alternatively, you can use repetative sampling for periodic waves. You'll get better resolution, but you'll lose visibility for any transient behavior.
And, of course, you need a graphical interface that will display the measured data as a waveform on the PC screen.
All-in-all building a PC digital oscilloscope from scratch is a very complex task. Unless you are an expert with high speed digital and analog circuits, your chances of success are not high.