Frequency detector for a phase locked loop system

摘要

A frequency detector circuit is arranged to detect a frequency difference between a clock signal and a reference clock signal. The frequency detector circuit includes four flip-flop circuits and a clear logic circuit. The clear logic circuit is arranged to clear selected flip-flop circuits. Two of the flip-flop circuits are arranged to detect two consecutive transitions in the clock signal without a clearing signal to provide a DOWN signal. The other two flip-flop circuits are arranged to detect two consecutive transitions in the reference clock signal without a clearing signal to provide an UP signal. The average of the UP and DOWN signals over a time interval corresponds to the difference in frequency between the clock signal and the reference clock signal. The UP and DOWN signals provide signals that may be employed by a charge pump circuit in a phase-locked-loop system to adjust the frequency of a VCO.