PLL is an electronic circuit that generates a constant waveform at a fixed frequency, accordingly to a reference signal given to it. Typical applications include clock signal generation, clock signal division in order to generate a clock signal at a lower frequency (frequency divider), data modulation and data demodulation.
On PC motherboards this circuit is typically used to generate the several different clock signals required by the several circuits located on the motherboard, being generically called ”clock generator“.
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Figure 1: Clock generator (PLL) used on a motherboard. The circuit on this picture is an ICS 954103EF.
PLL is a closed-loop circuit (hence ”loop“ on its name) meaning that is uses the signal found on its output in order to check if the signal frequency is correct or not, correcting itself, if necessary.