Abstract: A phase-sensitive detector has its reference signal modified in a manner to ensure that the output of the rectifier is substantially insensitive to harmonic frequencies of the reference signal. To achieve this the reference signal is used to generate a low-distortion sinusoidal control signal of the same frequency as the reference signal. The control signal controls a switching signal generator to generate a switching signal for repetitively switching the signal being rectified. The switching signal has spaces of interval T1 and marks of interval T2 given byT1=To/(1+kVp)T2=To/(1-kVp)where Vp is the mean value of the control signal during the relevant interval and To and k are constants chosen so that the output of the rectifier is substantially insensitive to harmonic frequencies of the reference signal.

Abstract: A phase-sensitive detector circuit comprises a pair of field-effect transistors switched in antiphase and connected at a switching point. The switching point is supplied with a signal to be detected through an a.c. coupling. One of the transistors is connected to the input of an operational amplifier having a feedback path designed to maintain its input as a virtual earth. A compensation circuit connected from the switching point to the input side of the a.c. coupling provides a compensation signal to cancel the effect of any offset voltage in the operational amplifier. The other switching transistor may be grounded or else may be connected to a second operational amplifier whose input is also maintained as a virtual earth by a feedback path. In the latter case the output from the circuit is taken differentially from the operational amplifier.