A voltage controlled oscillator or VCO is an oscillator that can be frequency controlled by an external voltage. The external voltage is called a tuning voltage and it is typically connected to a voltage variable capacitor (varactor) within the resonant oscillator circuit.

As the voltage changes, the capacitance of the varactor changes which, in turn, dithers the resonant frequency of the oscillator. This may sound simple but there are many constraints to good VCO design including spectral purity, output power, temperature behavior, tuning sensitivity and noise immunity. All of these constraints depend on the customer's specifications dictated by the application or end-use of the product.

For instance, high spectral purity is important in high data rate systems or communication systems where reciprocal mixing may be a problem. Reciprocal mixing occurs when the phase noise skirt of the local oscillator VCO down-converts environmental interferers which may desensitize a receiver. Temperature behavior is usually critical for airborne or remote applications such as radar or point-to-point radio base stations.