Thanks to all who have responded to this post.  I still don't know the final
answer, but you have given me some new ways to think about the problem, and
new ideas to research.

To answer the questions regarding things I left out:

        1)  The interfering signal is a dirty sine wave, and changes    slowly,
typically over a very limited range from its starting   frequency.

        2)  The signal of interest occurs <4 times per minute.

        3) This is a relatively low volume requirement, so parts cost is        not too
great of an issue.

        4) I perhaps should have mentioned that there is also present   random
noise, but at a considerably lower amplitude than the other     2 signals.

        5) Finally, the system must also work in the rare case where there      is no
interfering signal.

Thanks again for your input.

        Foster

=======================================================================
ORIGINAL POST FOLLOWS:
=======================================================================
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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