Foster wrote:
>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.
.........


Hi Foster, I have several ideas in mind - involving doing this in
software on a PIC. With an '877 running at 20 mhz, you can do A/D
sampling at 40 khz max. This would give you 4 samples/period with
your fastest signals, and should just about work.

1 - people have done FFTs on the PIC, but it looks like a lot of
    work, and you probably cannot get more than a few full calculations
    per second.

2 - since your interferring signal is always present, it looks like
    you could probably use an LMS adaptive filter, which would adapt
    to and reject the interference, but pass the target signal.

3 - if the signals are both sinewaves and the interfering signal is
    not doing any fancy phase jumps, it may be possible to simply
    lock onto the interfering signal with time sampling, and
    subtract it away from the composite target signal+interference
    to reveal the target signal alone.

With 20 mhz xtal and 40 khz A/D sampling, you have 200 intructions
to burn between samples, and you might be able to get away with doing
these kind of jobs.

However, since the PICs may not be fast enough to stay up with 40 khz
sampling rate, I was also thinking about the possibility of using a
Scenix, but those chips probably don't have enough RAM to do the job.

All in all, I think an adaptive filter, #2, might be something to
look at, short of going to a DSP chip.

best regards,
- dan michaels
www.oricomtech.com
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