This is a very good point. I think it is really important to understand the whole system end to end. What is generating these pulses? Do you have any control over the shape of the pulses? There are many techniques which allow filtering of known pulse shapes without degrading the rise-time (such as matched filtering and pulse coding) Sean On Tue, Dec 13, 2011 at 8:50 AM, Matt Bennett wrot= e: > On Mon, December 12, 2011 1:03 pm, Electron wrote: >> >> Hello, >> I have to get the *exact* timing (30ns resolution) of some low frequency >> (<250Hz) pulses >> via an input capture module, however I'd like to filter the signal comin= g >> in, as it will >> contain a lot of high frequency noise, but filtering via a capacitor >> introduces a phase >> delay, which I can't even compensate (by frequency) as the phase delay >> changes with input >> signal amplitude. >> >> What is the way You'd approach this problem? I thought about using a zen= er >> to clip the >> signal and thus work on a known amplitude, but I suspect it will introdu= ce >> errors too. >> >> On the software side, I could make sure that the input stays high for >> enough time, but >> it's pretty overkill, and I'd like to keep it as last option. If it coul= d >> be done [EE] >> then it would be better! >> >> With kind regards, >> Mario > > A very important aspect to this problem that I don't see discussed so far= , > is that the risetime of your signal and the bandwidth you let through are > completely related. =A0If the pulse is Gaussian shaped, the relationship > between risetime and bandwidth is B=3D0.35/Tr- so if you want a 30ns > risetime, the bandwidth of your input filter has to be greater than about > 12MHz. =A0Less than that, and your measurement is not going to be very > repeatable. (in other words, if you put a signal with a 1 femtosecond ris= e > time into a 12 MHz low pass filter, the rise time would be about 30ns.) > > Matt Bennett > Just outside of Austin, TX > 30.51,-97.91 > > The views I express are my own, not that of my employer, a large > multinational corporation that you are familiar with. > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .