A voltage controlled oscillator (VCO) is an oscillator in which the frequency tuning is performed by a capacitance diode. The VCO is controlled by a variable DC voltage. This is applied to the capacitance diode, which thereby changes its capacitance.
The capacitance diode is part of the oscillating circuit capacitance and causes a frequency changewithin the tunable frequency band due to the applied control voltage. The ratio between the highest and lowest oscillator frequencies results from the parallel connection of the oscillating circuit capacitor and the capacitance diode, namely when the latter has its highest and lowest capacitance.
VCOs are also called voltage-to-frequency converters and are used in radio and television sets, satellite receivers and other RF receiving equipment. Since the characteristic values for frequency accuracy and frequency stability of voltage-controlled oscillators often do not meet the requirements, there are various variants in which a crystal provides better characteristic values. One example is the Voltage Controlled Crystal Oscillator( VCXO).