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

["Samuel A. Falvo II" <sam.falvo@falvotech.com>]

On Tuesday 23 November 2004 04:50 pm, Daniel Nilsson wrote:
> 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.

This quantifies exactly what I was saying.  In order to capture those 
nice, clean, sharp edges, you need a wider scope bandwidth to grab the 
odd harmonics.  And you are correct; the sharper/steeper the edges, the 
wider the bandwidth required.

But if you measure a 250kHz square wave, and the analog bandwidth of the 
input section to the scope is only 250kHz, then what you'll end up 
finding on the screen is a 250kHz sine wave -- just the fundamental, 
with no harmonics (or, at least, substantially attenuated harmonics).

It's a handy formula to have -- however, I'm curious though: where does 
the factor of 0.35 come from?

--
Samuel A. Falvo II