phase locked loop

[sona]

A phase-locked loop or phase lock loop (PLL) is a control system that
generates a signal that has 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 is matched
to the reference in both frequency and phase. A phase-locked loop is
an example of a control system using negative feedback.


http://electronicstopics.blogspot.com/2008/10/phase-locked-loop.html