A receiver used in a system having a transmitter (10) that sends an exciting signal to a remote transponder (18) is configured to receive a relatively weak signal from the transponder and to cancel out relatively high level interference from the transmission itself. A received frequency shift keying signal is fed to a tuned amplifier (22) for preliminary analog signal conditioning, and thence fed to an analog to digital converter (24). The output of the analog to digital converter is integrated (26) over a selected number of clock pulses and then dumped into a quarter rate quadrature demodulator (60) that multiplies the integrated signal by the same signal delayed by one clock interval. Undesired artifacts generated in the demodulator (60) are removed by a cascade of comb filters (70), and the demodulated digital output is available as the desired output of the receiver. The several clock signals of the system are all referenced from a basic exciter signal that itself gives rise to the transmitted interference signal so that the interference and all timing signals are approximately synchronous.