Mr. De Boer -

Phase Locked Loop (PLL) is a feedback system that allows you to
synchronize two processes that may fall out of step with each other.

A PLL system looks at the outputs of two systems, say, the frequency
at which synchronization holes in an Ampico master roll and that of a
roll punch machine, pass by their sensors. The PLL will develop it's
own output that is constant if the two input values are identical. In
this situation, the PLL is said to be LOCKED, and no punch system
adjustment is necessary.

When the two inputs are not identical and thus out of PHASE, the PLL
attempts to re-capture the lock by making changes to it's output, plus
or minus, that allows ancillary feedback LOOP controlling mechanisms
to re-adjust the frequency or speed of one of the input systems to
restoring their synchronous or "locked" state.

In modern applications, PLL circuits are widely used in FM demodulation
circuits, data synchronizers, modems and tone decoders, to name a few.

Karl Ellison
Salem, Massachusetts