US Patent 7378880 Frequency comparator

A frequency comparator comparing frequencies of a first clock signal and a reference clock signal. The frequency comparator includes a phase-frequency detector and a comparison module. The phase-frequency detector receives the first clock signal and the reference clock signal, and outputs an up clock signal and a down clock signal. The pulse-width difference between the up clock signal and the down clock signal corresponds to the phase difference between the first clock signal and the reference clock signal. The comparison module compares the frequencies of the first clock signal and the reference clock signal based on how many times the pulse width of the up clock signal is larger or shorter than that of the down clock signal in a predetermined period.