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 coming
> 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 zene=
r
> to clip the
> signal and thus work on a known amplitude, but I suspect it will introduc=
e
> 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 could
> 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.  If 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.  Less than that, and your measurement is not going to be very
repeatable. (in other words, if you put a signal with a 1 femtosecond rise
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.
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