A voltage-controlled oscillator (VCO) is an electronic oscillator whose output frequency is proportional to its input voltage. An oscillator produces a periodic AC signal, and in VCOs, the oscillation frequency is determined by voltage.
Oscillators work by transferring energy back and forth from one form to another. One way to do this is with an LC circuit, where energy moves between the inductor (L) and the capacitor (C). The capacitor stores energy in the form of an electric field between its plates and discharges through the inductor, which then stores that energy in the form of a magnetic field. The inductor then charges the other plate of the capacitor and the process starts again, but with current now flowing in the opposite direction. The frequency at which this oscillation occurs is the resonant frequency, which is inversely proportional to √LC.
A voltage-controlled oscillator can be constructed by using a varactor diode as a voltage-controlled capacitor. As the reverse bias voltage across the varactor diode changes, so does its capacitance, and therefore so does its frequency.
VCOs are an important part of a phase-locked loop (PLL), which is a control system that generates a signal with a fixed relation to the phase of a “reference” signal. PLLs have a wide variety of uses in radio, telecommunications, computers, and other electronic applications. VCOs can also have other uses in frequency and phase modulation, and have applications such as function generators and synthesizers.