Edward Cheung wrote
...
>
>You may want to use a PIC controlled bandpass filter and sweep
>the pass band around.  Have the PIC output a frequency that is multiplied
>by the incoming signal using a 4 quadrant multiplier.  Low pass filter the
>output of the multiplier and peak detect the result.  This can serve
>as the amplitude of your signal.
>
 I'm not sure about the implementation, but what about a switched capacitor
filter for the bandpass (just a suggestion, since I#ve never worked with
either analog multipliers or switched cap filters) ? Of course this will
(like the multiplier setup) only be feasible if your input signal is
constant during the time needed to sweep the filter.

Just my .02 $

Bye
  Markus