Patent ID: 7508281

Claim:
A frequency synthesizer comprising: a voltage controlled oscillator that generates a frequency signal according to an input control voltage; a frequency divider that divides the generated frequency signal; an A/D converter that performs analog-to-digital conversion of the divided signal; a phase comparator that compares a phase of the A/D-converted signal with a phase of a reference signal to detect a phase difference, and outputs a phase difference signal according to the phase difference; a D/A converter that performs digital-to-analog conversion of the phase difference signal; and a loop filter that cancels a high-frequency noise, wherein the phase comparator includes, as an automatic gain control circuit, a multiplier provided at a subsequent stage of the A/D converter and an amplitude information detector that detects amplitude information for controlling a gain of the multiplier, and unlock detection means that monitors a value of the amplitude information and detects an unlock state when the value falls within a specific range.