A phase-locked loop or phase lock loop (PLL)'s a control system that generates a signal that's a fixed relation to the phase of a "reference" signal. A phase-locked loop circuit responds to both the frequency and the phase of the input signals, automatically raising or lowering the frequency of a controlled oscillator until it's matched to the reference in both frequency and phase. A phase-locked loop's an example of a control system using negative feedback. In simpler terms, a PLL compares the… (More on Pll)