PWM Demodulator

:? :idea: :?:
I need a PWM demodulator circuit to extract signal from following:
Carrier freq. = 20Khz
Carrier waveform = square
modulating freq. = 33Hz to 300Hz
modulating waveform = squre pulses
:cry:

A PWM demodulator is an integrator/ low pass filter , ur max mod freq is 300Hz and carrier 20K, so i would consider a low pass filter , of around say 3Khz