A signal having a desired frequency may be synthesised by a frequency synthesiser; frequency synthesisers have a local oscillator (L.O.) and may include a programmable or other divider, and the output of the local oscillator is locked in-phase to a reference source (typically a crystal type device the output of which may be divided by a reference divider). The local oscillator typically operates at several Megahertz or even Gigahertz while the frequency of the reference source is usually of the order of a few Kilohertz. Divided signals can then be phase compared in a phase detector which gives an output indicative of the phase imbalance of the divided signals. This allows frequency comparison by a digital function and so provides phase up or phase down error signal for use in controlling the local oscillator via an alteration of the applied voltage to a varactor diode.
A particular end product of such a frequency synthesiser is in the tuning circuit of a radio or television receiver. Conventionally, the phase detector is part of a phase and frequency comparator including a digital phase detector and an analogue phase detector. The digital detector has advantages in high frequency applications in which output pulses produced thereby are used to drive a charge pump circuit feeding or draining an integrating capacitor whose voltage is used to control a voltage dependant resonance element, such as a varactor diode, of a voltage controlled oscillator (VCO).
Digital phase detectors are inherently noisy and such method of VCO control can lead to considerable jitter on the output of the synthesiser.