Abstract: A circuit and method for detecting and determining the magnitude of either a synchronous frequency signal whose phase relationship to a reference signal is known or unknown or a non-synchronous signal near the reference frequency is based on using a pair of synchronous detectors one of which develops a COS Output and the other of which develops a SIN Output. The input sides of both synchronous detectors are connected to receive the signal being processed. The synchronous detector which develops the COS Output is connected to a first reference signal and the other synchronous detector input is connected to a second reference signal having a plus 90 degree phase relationship with the first reference signal. The COS Output and SIN Output are fed through a chopper analog switch controlled by a chop frequency and then to a RMS detector which develops the desired output.