Lock onto the phase. The frequency will follow.

A phase-locked loop, or PLL, is a feedback system that synchronizes the phase and frequency of an oscillator with a reference signal. A basic PLL contains a phase detector, loop filter, and voltage-controlled oscillator. The phase detector compares the reference with the oscillator output, producing an error signal that adjusts the oscillator until the two signals remain locked together. Once locked, the PLL can track changes in the reference while rejecting unwanted variations according to its loop characteristics. Frequency dividers may also be added to generate frequencies that are multiples of the reference. PLLs are used in radio communications, frequency synthesis, clock generation, and signal demodulation.