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
