A voltage-controlled oscillator is an electronic device that receives a voltage input and then outputs a moving signal with a frequency based on that input. With most voltage-controlled oscillators, the output frequency increases as the input voltage increases.
The function that relates output frequency to input voltage is most commonly linear, but it can also be defined by some other mathematical relationship. Voltage-controlled oscillators that have linear control parameters are sometimes referred to as voltage-to-frequency converters.
The two main types of voltage-controlled oscillators are linear or harmonic oscillators and relaxation oscillators. Linear voltage-controlled oscillators output sinusoidal, or curved, waveforms, while relaxation oscillators emit triangular or saw-tooth waveforms.
Voltage-controlled oscillators are used in several applications, including phase locked loops, function generators and synthesizers.