PLL

There's a good article about this in Wikipedia.

Basically a phase lock loop is a circuit used to compare a signal frequency with a known stable frequency.

A phase detector is used to compare the two frequencies and the output is used to alter the voltage controlled frequency oscillator to match.

So for example, a highly stable fixed frequency crystal oscillator can be used as the reference with a division ratio to obtain the frequency required and the PLL will change the voltage controlled oscillator frequency until it matches the reference frequency...

This gives the voltage controlled oscillator output the same degree of stability as the reference oscillator.

It can be used for many other uses as well.

John.