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
