An Improved “Lock-In” Amplifier

Abstract

An improved “lock‐in” amplifier or synchronous switch is described. A sinusoidal reference voltage drives a square‐wave generator, the output of which is applied to the anodes of a pair of diodes. The input is applied to the cathodes of the pair of diodes. The dc voltage developed between the two cathodes is observed on a vacuum‐tube voltmeter. This dc output voltage is directly proportional to the signal voltage, and is, to a large degree, independent of the general noise level at the input, the reference voltage amplitude, and the tube parameters.