This circuit passes the input signal at unity gain, either unchanged or inverted, depending on the digital control input. A buffered input is shown, since low-source impedance is essential. Gain can be added by modifications to the feedback networks. Signals up to 100kHz can be handled with 20.0-V pk-pk, output. The circuit becomes a phase detector when driving the digital control input with a reference phase at the same frequency as the input signal;
Balanced-modulator - schematic

the average de output is proportional to the phase difference, with 0 V at + 90°. By connecting the output to a comparator, which in tum drives the digital control, a synchronous full-wave rectifier is formed. With a low-frequency input signal and a high-frequency digital control signal, a balanced (suppressed carrier) modulator is formed.

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