A voltage-controlled oscillator (VCO) is an electronic circuit that generates a periodic signal, typically a sine or square wave, whose frequency is determined by an input voltage. This means that by changing the voltage applied to the VCO, you can control the frequency of the output signal. VCOs are widely used in various applications, including communication systems and signal processing. VCOs are essential components in devices like phase-locked loops (PLLs) and frequency modulation (FM) transmitters. They help in synthesizing frequencies and can be found in radio transmitters, synthesizers, and telecommunication equipment. Their ability to produce a range of frequencies makes them versatile in electronic design.