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

[Lars Segerlund <lars.segerlund@comsys.se>]

 Like the sampled signal being a sinus wave :-) .. and not counting the samples to end up fx. at 0 ( 0. 180 degrees ).

 The nyquist theorem only states the MINIMUM speed required to sample the signal :-) ... not what you actually need ...

 / regards, Lars Segerlund.


On Wed, 24 Nov 2004 10:20:00 -0500
"Peter J. Desnoyers" <pjd@cs.umass.edu> wrote:

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