A voltage controlled oscillator (VCO) is an oscillator with its output frequency determined by a voltage input. VCOs are commonly used to generate periodic signals such as sine waves, triangle waves, and square waves. VCOs offer a wide range of frequency modulation (FM), allowing them to be used in both frequency modulation synthesis and phase modulation synthesis. They are also commonly used to provide modulation sources for audio frequency modulations in synthesizers, as well as in control systems such as frequency synthesizers. VCOs typically have a voltage range of 1 to 10 volts. The frequency range of a typical VCO is from 1 Hz to 100 kHz, although some may be able to reach much higher or lower frequencies, depending on how they were designed.