I don't mind if it's a band-pass or low pass filter (although I think low-pass will give a better result). Input operating frequency range is 100Hz to 1kHz. Input amplitude will be about 1Vp-p and although it will be a bit noisy and not very square-wave-ish it'll be good enough to feed into a comparator to get a decent square-wave should that be required.
Basically I need a design that will take a noisy square-wave output from a strain gauge amplifier, determine the frequency of said square-wave and filter out the high frequency noise and harmonics leaving a sinewave.
EDIT - the end game is to measure the amplitude of the squarewave accurately - by filtering-in the fundamental frequency it becomes easier to measure because the squarewave amplitude is very accurately linked to the sine amplitude (4/Pi from memory)
This is for a test box where the strain gauge (connected to the amplifier) is supplemented with a FET switching a large value resistor connected across the strain gauge - this simulates a small but predictable amount of strain. The output from the amplifier needs to be converted to a sinewave for measurement purposes. I'm aware that this signal could be taken into a PC running a Fourier analysis on it but it's not always convenient to have a PC handy.
I suspect, for this tracking filter a switched-cap filter like the MF10 (or based on similar) will be required and a phase-locked-loop to track the actual input frequency to generate a clock at a much higher rate.
Currently, I have a 12th order low-pass filter that is good for a small range of frequencies around 600Hz (with minor adjustments) but this is no good for lower or higher frequencies.
Please let me know if there is information missing that would help answers. I am looking for quite high precision and if it needs a select-on-test components that's not a problem.