Voltage-controlled oscillator
Encyclopedia
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).
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.
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.
A voltage-controlled capacitor is one method of making an LC oscillator vary its frequency in response to a control voltage. Any reverse-biased semiconductor
diode
displays a measure of voltage-dependent capacitance and can be used to change the frequency of an oscillator by varying a control voltage applied to the diode. Special-purpose variable capacitance varactor diodes are available with well-characterized wide-ranging values of capacitance. Such devices are very convenient in the manufacture of voltage-controlled oscillatorsA voltage-controlled inductor would be in principle as useful, but such devices are unsatisfactory at the frequencies usually desired. 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
.
The frequency of a ring oscillator
is controlled by varying either the supply voltage or the capacitive loading on each stage.
(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:
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
.
Analog applications such as frequency modulation
and frequency-shift keying
often need to control an oscillator frequency with an input — a voltage-controlled oscillator (VCO). The functional relationship between the control voltage and the output frequency may not be linear. Over small ranges, the relationship is approximately linear, and linear control theory can be used.
There are devices called voltage-to-frequency converters (VFC). These devices are often designed to be very linear over a wide range of input voltages.
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.
VCOs generally have the lowest Q-factor of the used oscillators, and so suffer more jitter than the other types. The jitter can be made low enough for many applications (such as driving an ASIC), in which case VCOs enjoy the advantages of having no off-chip components (expensive) or on-chip inductors (low yields on generic CMOS processes). These oscillators also have larger tuning ranges than the other kinds, which improves yield and is sometimes a feature of the end product (for instance, the dot clock on a graphics card which drives a wide range of monitors).
Voltage-to-Frequency converters are voltage-controlled oscillators, with a highly linear relation between applied voltage and frequency. They are used to convert a slow analog signal (such as from a temperature transducer) to a digital signal for transmission over a long distance, since the frequency will not drift or be affected by noise. VCOs may have sine and/or square wave outputs. Function generators are low-frequency oscillators which feature multiple waveforms, typically sine, square, and triangle waves. Monolithic function generators are voltage-controlled. Analog phase-locked loops typically contain VCOs. High-frequency VCOs are usually used in phase-locked loop
s for radio receivers. Phase noise is the most important specification for them. Low-frequency VCOs are used in analog music synthesizers. For these, sweep range, linearity, and distortion are often most important specs. Since music synthesis is nowadays done digitally, the market for audio-frequency VCOs has largely disappeared.
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.
Electronic oscillator
An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...
designed to be controlled in oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
by a voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
input. The frequency of oscillation is varied by the applied DC voltage, while modulating
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
signals may also be fed into the VCO to cause frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
(FM) or phase modulation
Phase modulation
Phase modulation is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.Unlike its more popular counterpart, frequency modulation , PM is not very widely used for radio transmissions...
(PM); a VCO with digital pulse output may similarly have its repetition rate (FSK, PSK) or pulse width
Pulse-width modulation
Pulse-width modulation , or pulse-duration modulation , is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches....
modulated (PWM).
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
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
displays a measure of voltage-dependent capacitance and can be used to change the frequency of an oscillator by varying a control voltage applied to the diode. Special-purpose variable capacitance varactor diodes are available with well-characterized wide-ranging values of capacitance. Such devices are very convenient in the manufacture of voltage-controlled oscillatorsA voltage-controlled inductor would be in principle as useful, but such devices are unsatisfactory at the frequencies usually desired. 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
Current source
A current source is an electrical or electronic device that delivers or absorbs electric current. A current source is the dual of a voltage source. The term constant-current sink is sometimes used for sources fed from a negative voltage supply...
) are used. See Function generator
Function generator
A function generator is a piece of electronic test equipment or software used to generate different types of electrical waveforms over a wide range of frequencies. These waveforms can be either repetitive or single-shot, in which case some kind of triggering source is required...
.
The frequency of a ring oscillator
Ring oscillator
A ring oscillator is a device composed of an odd number of NOT gates whose output oscillates between two voltage levels, representing true and false...
is controlled by varying either the supply voltage or the capacitive loading on each stage.
Voltage-controlled crystal oscillators
A voltage-controlled crystal oscillatorCrystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...
(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
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
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
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
of the crystal. A smaller range of voltage control then suffices to stabilize the oscillator frequency in applications where temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
varies, such as heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
buildup inside a transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
.
VCO time-domain equations
-
-
-
is called the oscillator gain. Its units are hertz per volt.
is the symbol for the time-domain waveform that is the VCO's tunable frequency component.
is the symbol for the time-domain waveform that is the VCO's output phase.
is the time-domain symbol of the control (input) voltage of the VCO; it is sometimes also represented as 
-
-
VCO freq-domain equations
Analog applications such as frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
and frequency-shift keying
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
often need to control an oscillator frequency with an input — a voltage-controlled oscillator (VCO). The functional relationship between the control voltage and the output frequency may not be linear. Over small ranges, the relationship is approximately linear, and linear control theory can be used.
There are devices called voltage-to-frequency converters (VFC). These devices are often designed to be very linear over a wide range of input voltages.
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 material, 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
Clapp oscillator
The Clapp oscillator is one of several types of electronic oscillator constructed from a transistor and a positive feedback network, using the combination of an inductance with a capacitor for frequency determination, thus also called LC oscillator.It was published by James Kilton Clapp in 1948...
and Colpitts
Colpitts oscillator
A Colpitts oscillator, invented in 1920 by American engineer Edwin H. Colpitts, is one of a number of designs for electronic oscillator circuits using the combination of an inductance with a capacitor for frequency determination, thus also called LC oscillator...
oscillators. The more widely used oscillator of the two is Colpitts and these oscillators are very similar in configuration.
VCOs generally have the lowest Q-factor of the used oscillators, and so suffer more jitter than the other types. The jitter can be made low enough for many applications (such as driving an ASIC), in which case VCOs enjoy the advantages of having no off-chip components (expensive) or on-chip inductors (low yields on generic CMOS processes). These oscillators also have larger tuning ranges than the other kinds, which improves yield and is sometimes a feature of the end product (for instance, the dot clock on a graphics card which drives a wide range of monitors).
Applications
VCOs are used in:- Electronic jamming equipment
- Function generatorFunction generatorA function generator is a piece of electronic test equipment or software used to generate different types of electrical waveforms over a wide range of frequencies. These waveforms can be either repetitive or single-shot, in which case some kind of triggering source is required...
s, - The production of electronic musicElectronic musicElectronic music is music that employs electronic musical instruments and electronic music technology in its production. In general a distinction can be made between sound produced using electromechanical means and that produced using electronic technology. Examples of electromechanical sound...
, to generate variable tones, - Phase-locked loopPhase-locked loopA phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
s, - Frequency synthesizerFrequency synthesizerA frequency synthesizer is an electronic system for generating any of a range of frequencies from a single fixed timebase or oscillator. They are found in many modern devices, including radio receivers, mobile telephones, radiotelephones, walkie-talkies, CB radios, satellite receivers, GPS systems,...
s used in communication equipment.
Voltage-to-Frequency converters are voltage-controlled oscillators, with a highly linear relation between applied voltage and frequency. They are used to convert a slow analog signal (such as from a temperature transducer) to a digital signal for transmission over a long distance, since the frequency will not drift or be affected by noise. VCOs may have sine and/or square wave outputs. Function generators are low-frequency oscillators which feature multiple waveforms, typically sine, square, and triangle waves. Monolithic function generators are voltage-controlled. Analog phase-locked loops typically contain VCOs. High-frequency VCOs are usually used in phase-locked loop
Phase-locked loop
A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
s for radio receivers. Phase noise is the most important specification for them. Low-frequency VCOs are used in analog music synthesizers. For these, sweep range, linearity, and distortion are often most important specs. Since music synthesis is nowadays done digitally, the market for audio-frequency VCOs has largely disappeared.
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 noisePhase noise
Phase noise is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a waveform, caused by time domain instabilities...
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.
See also
- VFO
- VCFVoltage-controlled filterA voltage-controlled filter is a filter whose operating characteristics can be controlled by means of a control voltage applied to one or more inputs...
- VCAVariable-gain amplifierA variable-gain or voltage-controlled amplifier is an electronic amplifier that varies its gain depending on a control voltage .VCAs have many applications, including audio level compression, synthesizers and amplitude modulation....
- LFO
- modular synthesizerModular synthesizerThe modular synthesizer is a type of synthesizer consisting of separate specialized modules connected by wires to create a so-called patch. Every output generates a signal – an electric voltage of variable strength...
- Numerically controlled oscillator (Also "digitally controlled oscillator" or DCO)
- Phase-locked loopPhase-locked loopA phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...