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A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency modulation (FM) or phase modulation (PM); a VCO with digital pulse output may similarly have its repetition rate (FSK, PSK) or pulse width modulated (PWM).

Voltage-controlled oscillator schematic - audio

Contents

Types of VCOs

VCOs can be generally categorized into two groups based on the type of waveform produced: 1) harmonic oscillators, and 2) relaxation oscillators.

Harmonic oscillators generate a sinusoidal waveform. They consist of an amplifier that provides adequate gain and a resonant circuit that feeds back signal to the input. Oscillation occurs at the resonant frequency where a positive gain arises around the loop. Some examples of harmonic oscillators are crystal oscillators and LC-tank oscillators. When part of the resonant circuit's capacitance is provided by a varactor diode, the voltage applied to that diode varies the frequency.

Relaxation oscillators can generate a sawtooth or triangular waveform. They are commonly used in monolithic integrated circuits (ICs). They can provide a wide range of operational frequencies with a minimal number of external components. Relaxation oscillator VCOs can have three topologies: 1) grounded-capacitor VCOs, 2) emitter-coupled VCOs, and 3) delay-based ring VCOs. The first two of these types operate similarly. The amount of time in each state depends on the time for a current to charge or discharge a capacitor. The delay-based ring VCO operates somewhat differently however. For this type, the gain stages are connected in a ring. The output frequency is then a function of the delay in each of stages.

Harmonic oscillator VCOs have these advantages over relaxation oscillators.

  • Frequency stability with respect to temperature, noise, and power supply is much better for harmonic oscillator VCOs.
  • They have good accuracy for frequency control since the frequency is controlled by a crystal or tank circuit.

A disadvantage of harmonic oscillator VCOs is that they cannot be easily implemented in monolithic ICs. Relaxation oscillator VCOs are better suited for this technology. Relaxation VCOs are also tunable over a wider range of frequencies.

Control of frequency in VCOs

For high-frequency VCOs the voltage-controlled element is commonly a varicap diode connected as part of an LC tank circuit. For low-frequency VCOs, other methods of varying the frequency (such as altering the charging rate of a capacitor by means of a voltage controlled current source) are used. See Function generator.

Voltage-controlled crystal oscillators

A voltage-controlled crystal oscillator (VCXO) is used when the frequency of operation needs to be adjusted only finely. The frequency of a voltage-controlled crystal oscillator can be varied only by typically a few tens of parts per million (ppm), because the high Q factor of the crystals allows "pulling" over only a small range of frequencies.

There are two reasons for using a VCXO:

  • To adjust the output frequency to match (or perhaps be some exact multiple of) an accurate external reference.
  • Where the oscillator drives equipment that may generate radio-frequency interference, adding a varying voltage to its control input can disperse the interference spectrum to make it less objectionable. See spread-spectrum clock generation.

A temperature-compensated VCXO (TCVCXO) incorporates components that partially correct the dependence on temperature of the resonant frequency of the crystal. A smaller range of voltage control then suffices to stabilize the oscillator frequency in applications where temperature varies, such as heat buildup inside a transmitter.

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VCO time-domain equations

\ f_{tuning}(t) = K_o \cdot \ v_{in}(t)
\int f_{tuning}(t)\,dt = \theta_{out}(t)
  • \ K_o is called the oscillator gain. Its units are hertz per volt.
  • \ f_{tuning}(t) is the symbol for the time-domain waveform that is the VCO's tunable frequency component.
  • \ \theta_{out}(t) is the symbol for the time-domain waveform that is the VCO's output phase.
  • \ v_{in}(t) is the time-domain symbol of the control (input) voltage of the VCO; it is sometimes also represented as \ v_{tune}(t)

VCO freq-domain equations

\ F_{tuning}(s) = K_o \cdot \ V_{in}(s)
\ {F_{tuning}(s) \over s} = \Theta_{out}(s)

VCO design and circuits

Tuning range, tuning gain and phase noise are the most important factors of the basic design of a VCO. Generally low phase noise is preferred in the VCO. The important elements that determine the phase noise of an oscillator are the transistor's flicker noise corner frequency, the loaded Q of the resonator and the final signal to noise ratio.

Most commonly used VCO circuits are the Clapp and Colpitts oscillators. The more widely used oscillator of the two is Colpitts and these oscillators are very similar in configuration.

Applications

VCOs are used in:

Voltage-Controlled Crystal Oscillator as a Clock Generator

A clock generator is an oscillator that provides a timing signal to synchronize operations in digital circuits. VCXO clock generators are used in many areas such as digital TV, modems, transmitters and computers. Design parameters for a VCXO clock generator are tuning voltage range, center frequency, frequency tuning range and the timing jitter of the output signal. Jitter is a form of phase noise that must be minimised in applications such as radio receivers, transmitters and measuring equipment.

The tuning range of a VCXO is typically a few ppm (parts per million) corresponding to a control voltage range of typically 0 to 3 volts. When a wider selection of clock frequencies is needed the VCXO output can be passed through digital divider circuits to obtain lower frequency(ies) or be fed to a PLL (Phase Locked Loop). ICs containing both a VCXO (for external crystal) and a PLL are available. A typical application is to provide clock frequencies in a range from 12 kHz to 96 kHz to an audio digital to analog converter.

Chips

  • LM231, LM331 are VCOs (Precision Voltage-to-Frequency Converters, NS).
  • VCOs can be found in the TTL 74124 Dual VCO and the CMOS 4046 PLL.

See also

External links


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