A basic clock circuit works by receiving an input frequency from a source and either distributing that frequency or generating new frequencies to send as outputs to other devices within the system. This can either be done using a Phase Locked Loop (PLL) or non-PLL based circuitry. The input to the clock can come from either a crystal or from another device within the system. A system signal is either single-ended or differential, which is defined by the source device. The main difference between PLL and non-PLL clocks is that the PLL uses a feedback loop to remove the time delay between the signal input and output, known as propagation delay. The feedback loop also allows the PLL to act as a phase detector to keep an oscillator in phase with an incoming frequency. PLL clocks are able to eliminate propagation delay, move the output phase with respect to the input phase, make duty cycle corrections, perform integer or fractional multiplication, and remove noise from the reference clock. PLL clocks are used when the system needs to minimize the propagation delay.