The control mechanism used in most frequency synthesizers. Operates by comparing the phase of a sample signal, usually the frequency-divided output of the RF oscillator, with a precision fixed reference frequency. The phase comparator output then drives a control loop which forces the RF oscillator frequency to be an exact multiple of the reference frequency. Changing the frequency divider ratio will lock the oscillator to a new frequency, changing the operating frequency of the synthesizer. See also Synthesizer.