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

[Daniel Nilsson <daniel@oden.homeip.net>]

On Tue, Nov 23, 2004 at 07:40:31AM -0800, Samuel A. Falvo II wrote:
> On Tuesday 23 November 2004 05:00 am, Xtian Xultz wrote:
> > this produtc says it makes 20M samples per second, but have a analog
> > bandwidth of 250k Hz. What is the greatest (in frequency) signal I can
> > measure with it? Someone knows?
> 
> If the analog bw is 250kHz, then you can expect accurate representation 
> of waveforms up to 50kHz (square waves will have harmonics all the way 
> out to and beyond 250kHz).  If you are just observing sine waves, then 
> 250kHz would be your top frequency.

Regarding square waves, the frequency of the square wave doesn't
matter from a measurement perspective. What matters is the rise and
falltime of the signal that you are measuring. To calculate the BW
based on the risetime a good approximation is BW=0.35/tr where tr is
the fastest out of rise or falltimes. For example, is you signal has a
10ns risetime, the bandwidth requirement would be 0.35/10e-9 =
35MHz.

Now this 35MHz bandwidth only allows you to measure a 10ns edge
correctly, if you are interested in any glitching/ringing or so you
need to oversample this signal. Depending on who you ask (scope
manufacturers are most conservative...) you get answers between 3 and
10 times. But somewhere in the 3-5 range i probably OK. So in reality
you asking for 5*35MHz=175MHz bandwidth to look at a digital signal
with 10ns edge rate (which is very slow with todays standards). The
sampling rate then needs to be at least twice of that ( > 350Ms/s ).

-- 
Daniel Nilsson