VCXO is a voltage controlled crystal oscillator in which the output frequency can be adjusted by varying the external control voltage across a variable capacitor (varactor diode) within the oscillator circuit. The associated change in frequency due to the change in control voltage is known as pullability. VCXOs are widely used in Telecommunications, Instrumentations, ADSL, and other RF electronic equipment where a stable but electrically tunable frequency is required.
The varactor diode is a semiconductor device that is designed to act as a variable capacitor when a voltage is applied to it. When used in series with a crystal, as shown in Fig (2). Changing the control voltage causes diode’s capacitance to change. This change in capacitance causes the total crystal’s on-board load capacitance to change and subsequently causes a change in crystal frequency.
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