Re: gEDA-user: Any DIY USB Scope project on schedule? Or some recommmendation?

["Peter J. Desnoyers" <pjd@cs.umass.edu>]

Charles Lepple wrote:
> The Nyquist theorem (20 MS/sec would give you a theoretical upper
> bandwidth of just under 10 MHz) assumes ideal sampling, and probably a
> bunch of other caveats that I can't remember now.
For a periodic signal that repeats exactly with a period t, and a 
maximum frequency f, you need 2ft samples to represent it.  That's not a 
sampling rate, but a total number of samples. And you don't need to get 
all the samples during a single period of the signal.

This is the basis of sub-nyquist sampling - if the analog bandwidth into 
the A/D, sampling jitter, and a few other things are good enough, then 
you can sample at 2f/N for N (+some delta) repetitions of the signal.

Basically you're doing what an analog scope does - you're capturing 
multiple iterations of the same signal, and averaging them together. And 
just like an analog scope, if there is any variation in the repetitions, 
it will show up as noise. (although sometimes the noise can be harder to 
interpret - I know from experience that certain digital scopes can show 
timing jitter as an impossibly high frequency noise on top of the 
signal, instead of the multiple traces you would see on an analog scope)

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 Peter Desnoyers                                  pjd@cs.umass.edu
 UMass Computer Science                           (617) 669-4728