A Voltage-Controlled Oscillator (VCO) is an electronic circuit that generates a periodic signal, typically a sine wave or square wave, whose frequency can be adjusted by varying the input voltage. VCOs are widely used in communication systems, such as in phase-locked loops and frequency modulation, to create signals that can be tuned to specific frequencies. The output frequency of a VCO is directly proportional to the input voltage, allowing for precise control over the signal's characteristics. This makes VCOs essential components in various applications, including synthesizers, radio transmitters, and signal processing devices, where frequency modulation is required.