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What Is a Voltage-Controlled Oscillator?

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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