A circuit for controlling an RF generator, the circuit including first and second heterodyne stages. The first heterodyne stage receives an input signal, which is based on a characteristic of an RF signal generated by the RF generator, and is configured to: mix the input signal with a first mix signal to generate a first heterodyne signal and to filter the first heterodyne signal through a low pass filter. The second heterodyne stage receives the filtered first heterodyne signal and is configured to: mix the filtered first heterodyne signal with a second mix signal to generate a second heterodyne signal and to filter the second heterodyne signal through a band pass filter. A detection stage converts the filtered second heterodyne signal to a DC signal, and a power control stage receives the DC signal and controls the RF signal in response to the DC signal.