A voltage-controlled oscillator (VCO) is an electronic oscillator whose frequency can be controlled by an input voltage. It is a key component in many electronic systems, particularly in communication systems, frequency synthesis, and modulation/demodulation circuits.

The VCO generates an output signal with a frequency that is directly proportional to the input voltage applied to its control input. By varying the control voltage, the frequency of the output signal can be adjusted over a certain range.

The basic configuration of a VCO typically consists of a resonant tank circuit, such as an LC (inductor-capacitor) or an RC (resistor-capacitor) circuit, and an amplifier. The tank circuit determines the resonant frequency of the oscillator, and the amplifier provides gain to compensate for energy losses and sustain oscillation.

The control voltage is typically applied to a varactor diode within the tank circuit. The varactor diode’s capacitance changes in response to the control voltage, which alters the resonant frequency of the tank circuit and thus the output frequency of the VCO.

VCOs find application in various electronic systems. For example, in communication systems, they are used for frequency modulation (FM) and frequency-shift keying (FSK). In frequency synthesis, VCOs are often used as the main source to generate signals at different frequencies for different purposes, such as local oscillators in radio receivers or as clock sources in microprocessors.

Overall, the voltage-controlled oscillator is a versatile component that provides a tunable output frequency based on the input voltage, making it an essential building block in many electronic circuits.