Hi Mario,

It is not possible to perform filtering without delay - however, what
you CAN do is make the delay predictable and then compensate for it.

In a linear filter, the delay does not depend on the signal amplitude
(maybe you were referring to feeding the output of the filter into a
digital gate?). It DOES, in general, depend on the signal frequency.
However, there is a class of filters called "linear phase" which have
a constant delay over frequency and so they do not distort signals
within their passband.

FIR discrete-time filters can be designed to be exactly linear phase
(this includes the method of recording the signal, taking an FFT,
multiplying by the desired frequency response, and then inverting the
FFT). IIR discrete-time filters (performed in real-time) and
continuous time filters cannot be perfectly linear phase but there is
an approximate design method which produces a Bessel filter (as Russel
pointed out here). This is the continuous-time filter type which has
the least possible amount of variation in its delay over frequency,
but still accomplishes some required filtering.

It would help greatly if you could more fully describe what you are doing.

Sean



On Mon, Dec 12, 2011 at 2:03 PM, Electron <electron2k4@infinito.it> 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 intr=
oduces a phase
> delay, which I can't even compensate (by frequency) as the phase delay ch=
anges 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 eno=
ugh 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
>
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