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A method for measuring frequency at a fixed sampling rate is described, and a variable-rate method is introduced. An algorithm based on a digital Fourier transform, which locks the sampling frequency in a microprocessor-based system to the fundamental of an incoming voltage waveform, is devised. The algorithm also intrinsically provides a measurement of the frequency. The algorithm is applied to the case of a volt/hertz relay.>
G. Benmouyal (1989) studied this question.