A PLL circuit, or Phase-Locked Loop, features a Voltage-Controlled Oscillator (VCO) whose output is constantly adjusted to match the phase and frequency of an input signal using feedback from a Phase Comparator. A PLL circuit is often used to synchronise (lock) RF receiver circuits to an incoming carrier signal. If a frequency divider n is introduced between the VCO output and the phase comparator, the PLL circuit output frequency will be fin/n, locked in phase to fin. If fin comes from a highly-stable fixed frequency oscillator and n is programmable, then the circuit forms the basis of an accurate Digital Frequency Synthesiser.