METHOD AND CIRCUIT FOR DETERMINING THE PRESENCE OR NO PRESENCE OF AT LEAST ONE FREQUENCY KNOWN VALUE IN AN INPUT SIGNAL COMPOSED OF MULTIPLE FREQUENCIES

摘要

A method and circuit for determining the presence or absence of a frequency of known value in an input signal composed of several frequencies employ a frequency generator, to which a stable reference signal is supplied, for generating two switching signals having a frequency which is the same as the known frequency and a phase difference of 90 DEG . The input signal is supplied to two mixers, to which the out-of-phase signals from the frequency generator are respectively supplied. The products of the two mixers are supplied to respective filters for filtering out ac components, and the respective amplitudes of the remaining dc voltage components of the respective mixed products are evaluated, the result of the evaluation indicating the presence or absence of the known frequency in the input signal.