English: Block diagram of a superheterodyne radio receiver using a frequency synthesizer. Older superheterodynes use a LC oscillator as the local oscillator which is tuned to different stations with a variable capacitor. However this type of oscillator's frequency is not very stable, causing the receiver to drift off the frequency of the station, Modern receivers use a master crystal oscillator which uses the vibrations of a quartz crystal to produce a very stable frequency. Since a crystal oscillator's frequency cannot be varied, to tune the receiver to different stations a digital circuit called a frequency synthesizer is used to generate different frequencies from the master oscillator signal. The synthesizer uses a phase locked loop so that the new frequency is locked to the master oscillator frequency. The frequency to be generated is specified by a digital "frequency control word" (FCW), which is generated by digital channel selection logic