The incoming signal is sampled by a multiplexer at four 90 deg intervals over its cycle. This results in detected audio samples of 0 deg, 90 deg, 180 deg and 270 deg at the mux output. Op-amp difference amplifiers subtract the 0/180 samples and the 90/270 samples to produce 2 audio samples that are 90 deg spaced. IQ signals will be generated for signals the the modulator frequency. Any strong off-channel signals that blast through the sampler appear at equal phases and amplitudes on the 4 mux outputs. The difference amplifiers subtract these unwanted equal-amplitude / equal phase spurious signals to produce a net value of zero at the I and Q outputs.