Voltage-Controlled Oscillator Characteristics & Applications

A voltage-controlled oscillator abbreviated as VCO is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillation frequency. Consequently, a VCO can be used for frequency modulation (FM) or phase modulation (PM) by applying a modulating signal to the control input. A VCO is also an integral part of a phase-locked loop. A voltage-to-frequency converter (VFC) is a special type of VCO designed to be very linear in frequency control over a wide range of input control voltages.

voltage-controlled oscillator

Design and Operation

An oscillator is a device that generates an alternating signal when it is energized. Electronic oscillators are powered by electricity. The operation is dependent on the design and as such we will first check of factors considered in the design. The following are key parameters of a voltage-controlled oscillator; tuning range, tuning gain, and phase noise. Tune gain is a measure of change of frequency of output signal compared to the reference level. Tuning range on the other hand is a measure of frequency scale that a signal can take. Phase noise is an undesirable parameter and should be as low as possible. The noise present in the control signal and the tune gains directly affect the phase noise. The relationship is that of direct proportional. Since high gain may be desired the signal noise is kept at the minimum for low phase noise.

Other important elements that determine the phase noise are; sources of flicker noise in the circuit, the output power level, and the loaded Q factor of the resonator. Low-frequency flicker noise affects the phase noise. This is because the flicker noise is heterodyned to the oscillator output frequency.  The flicker noise is a result of the non-linear transfer function of the device as it is active. The effect of flicker noise can be reduced with negative feedback that linearizes the transfer function, for example, emitter degeneration.

The two common types of VCO are an LC tank circuit and an Integrated circuit (IC). An LC tank has an inductor and capacitor connected to a voltage source. The oscillations are produced by electrical energy exchange between capacitor and inductor. An operational amplifier is usually used to amplify the output signal. The IC design of voltage-controlled oscillators usually produces square waves. It has a resistor and a capacitor and a voltage source that enables control of the output signal.


  1. The oscillator’s frequency is determined by the applied input voltage.
  2. VCOs have a lower Q factor compared to similar fixed-frequency oscillators, and so suffer more jitter.
  3. They have low phase noise. This is because of the tuning gain and noise present in the control signal.
  4. Easy to control output signal both in shape and frequency.


  • VCOs are used in function generators, phase-locked loops including frequency synthesizers used in communication equipment, and the production of electronic music, to generate variable tones in synthesizers.
  • Audio-frequency VCOs are used in analog music synthesizers. For these, sweep range, linearity, and distortion are often the most important specifications.
  • Voltage-to-frequency converters are voltage-controlled oscillators with a highly linear relationship between applied voltage and frequency.


voltage-controlled oscillators are important oscillators in frequency control which is important in the control of many electronic devices.

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