Varactor diodes are widely used within the RF design arena. They provide a method of varying he capacitance within a circuit by the application of a control voltage. This gives them an almost unique capability and as a result varactor diodes are widely used within the RF industry.
Although varactor diodes can be used within many types of circuit, they find applications within two main areas:
Voltage controlled oscillators, VCOs: Voltage controlled oscillators are used for a variety of applications. One major area is for the oscillator within a phase locked loop - this are used in almost all radio, cellular and wireless receivers. A varactor diode is a key component within a VCO.
RF filters: Using varactor diodes it is possible to tune filters. Tracking filters may be needed in receiver front end circuits where they enable the filters to track the incoming received signal frequency. Again this can be controlled using a control voltage. Typically this might be provided under microprocessor control via a digital to analogue converter.
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