COMPARATOR FOR DETERMINING BOTH THE MAGNITUDE AND THE SENSE OF DIFFERENCE IN THE FREQUENCIES OF TWO SIGNALS

摘要

1,021,409. Frequency comparison. STANDARD TELEPHONES & CABLES Ltd. Sept. 4, 1964, No. 36339/64. Heading H3A. A circuit for comparing the frequency of two signals F1, F2 applied at terminals 1, 2 and providing an indication of the magnitude and sense of the difference comprises a mixer 3 to which the signal F1 is applied via phase modulator 8 and multiplier 6 and to which the signal F2 is applied via a similar multiplier 7, an indicator 5 coupled to the mixer output and a switched feed-back path 9 for varying the phase change introduced by modulator 8 in accordance with the mixer output. With the switch 10 in its open position indicator 5 provides an indication which is a function of the frequency of the beat signal. In the closed position of the switch the feed-back signal advances the phase of signal F1 when the beat signal is positive-going and retards its phase when the beat signal is negative-going with the result that the beat signal is distorted to provide a mean signal value having one sign when signal F1 is higher in frequency than signal F2 and the opposite sign when the signal frequencies are reversed. The sense of the difference is then indicated by a centre-zero instrument.