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What Is a Voltage-Controlled Oscillator?

A Voltage-Controlled Oscillator (VCO) is an electronic device that generates a periodic signal whose frequency is directly influenced by an input voltage. The beauty of a VCO lies in its versatility; it's the heartbeat in radios, synthesizers, and telecommunication systems, adjusting frequencies with precision. Curious about how this component shapes modern technology? Dive deeper to uncover the VCO's pivotal role.
Joe Williams
Joe Williams

A voltage-controlled oscillator (VCO) uses an input voltage to determine its oscillation frequency. This can include the input of direct current (DC) with a fixed voltage to produce an oscillation with a fixed frequency. A voltage-controlled oscillator can also receive a changing voltage to produce an oscillation with a modulated frequency. Some oscillators of this type have a digital pulse output that can produce pulses with a modulated width. A VCO might be classified as a harmonic oscillator or relaxation oscillator, depending on the type of waveform it produces.

Harmonic oscillators produce waveforms with sinusoidal shapes. This type of voltage-controlled oscillator must include an amplifier to feed the signal back to the source of the input voltage, which creates a resonant frequency that produces a positive gain around the feedback loop and creates the sinusoidal waveform. A harmonic oscillator might also include a varactor diode that provides part of the capacitance for the resonant circuit. The oscillation frequency can then be varied according to the diode’s voltage input.


A relaxation oscillator produces a waveform with a triangular shape. This type of voltage-controlled oscillator contains three subtypes, including an emitter-coupled oscillator, a grounded-capacitor oscillator and a delay-based ring oscillator. The amount of time needed to charge the capacitor determines the oscillation frequency for the emitter-coupled oscillator and grounded-capacitor oscillator. The oscillation frequency of the delay-coupled oscillator depends on the amount of time it takes for the oscillator to attain each increase in gain.

Harmonic voltage-controlled oscillators have greater frequency stability than relaxation VCOs over changes in noise, power supply and temperature. The frequency control is also better in a harmonic VCO, since this type uses a separate circuit to control the frequency. The primary advantage of a relaxation VCO is its ability to be tuned over a wider frequency range than the harmonic type. A relaxation VCO also is better suited to use in integrated circuits.

A voltage-controlled oscillator frequently uses a voltage-controlled capacitor to control the output frequency. This type of capacitor contains a semiconductor diode with a capacitance that depends on its voltage. The oscillator’s output frequency can then be controlled by modifying the diode’s input voltage. A voltage-controlled capacitor is a convenient method of controlling the output frequency of a high-frequency VCO because voltage-controlled capacitors are readily available in a wide range of capacitances. A low-frequency VCO typically uses a voltage-controlled source of electrical current to vary the output frequency.

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