Frequency comparator

摘要

Two frequencies to be compared are applied to two inputs of a flip-flop. The output of the flip-flop is fed to a low pass filter and the output of the filter is a saw tooth voltage proportional to the phase difference between the two frequencies. Differentiation of the saw tooth voltage yields a voltage proportional to the difference between the two frequencies. A threshold circuit is provided for sensing the output voltage of the filter to control a flip-flop which in turn reverses the phase of one of the frequency signals and disconnects the output of the differentiator from a capacitive storage circuit.