A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency modulation (FM) or phase modulation (PM); a VCO with digital pulse output may similarly have its repetition rate (FSK, PSK) or pulse width modulated (PWM).
Read more about Voltage-controlled Oscillator: Types of VCOs, Control of Frequency in VCOs, Voltage-controlled Crystal Oscillators, VCO Design and Circuits, Applications, Voltage-controlled Crystal Oscillator As A Clock Generator