If you could synchronize your sampling to the zero crossings of the
interfering signal it would disappear.  I think filtering in the time
domain will be tough.

Sherpa Doug

> -----Original Message-----
> From: adastra [mailto:foster@ADASTRAN.COM]
> Sent: Monday, July 09, 2001 1:29 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [PIC]: Suggest an approach to this filtering problem?
>
>
> I need to measure the frequency (or period) of a sinewave
> signal in the
> range of 1KHz to 10KHz, which is only present for about 0.5 second.
>
> The trouble is that there is another signal present ALL of
> the time that is
> in the same frequency range and of approximately the same
> amplitude.  It
> seems I need to filter out the always-there signal so I can
> measure the
> intermittent one.  The exact frequency of the interfering
> signal is not
> known, and could be higher or lower than the signal of interest.  It
> probably must be determined before the filtering can occur,
> but that part
> shouldn't be too difficult since the interfering signal is
> there prior to
> the appearance of the signal of interest.
>
> I realize that if the signal-of-interest happens to be the same as the
> interfering signal, I probably cannot detect it, but if the
> filter is narrow
> enough, I can live with that.
>
> I thought I could do this with a couple of analog PLLs ahead
> of the PIC, but
> lately I am wondering if I need something more along the
> lines of a FFT
> system, or maybe a DSP chip.  I would really like to do
> everything in a
> 16F877 if possible.
>
> If anyone could point me in the right direction I would
> appreciate it very
> much.  Any and all suggestions welcome.
>
> Thanks, Foster
>
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