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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.
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Henry Cox (1953) studied this question.
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