Avoltage-controlled oscillator (VCO) is anelectronic oscillator whoseoscillationfrequency is controlled by avoltage input. The applied input voltage determines the instantaneous oscillation frequency. Consequently, a VCO can be used forfrequency modulation (FM) orphase modulation (PM) by applying amodulating signal to the control input. A VCO is also an integral part of aphase-locked loop. VCOs are used insynthesizers to generate awaveform whosepitch can be adjusted by a voltage determined by amusical keyboard or other input.
Avoltage-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.[1][2][3]
VCOs can be generally categorized into two groups based on the type of waveform produced.[4]
A voltage-controlled capacitor is one method of making an LC oscillator vary its frequency in response to a control voltage. Any reverse-biasedsemiconductor 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-capacitancevaractor diodes are available with well-characterized, wide-ranging values of capacitance. A varactor is used to change the capacitance (and hence the frequency) of an LC tank. A varactor can also change the loading on a crystal resonator and pull its resonant frequency.
The same effect occurs with bipolartransistors, as described by Donald[5] E. Thomas atBell Labs in 1954: with atank circuit connected to the collector and the modulating audio signal applied between the emitter and the base, a single-transistor FM transmitter is created.[6] Thomas worked with apoint-contact transistor, but the effect also works injunction transistors; applications includewireless microphones such as that patented byRaymond A. Litke in 1964.[7]
For low-frequency VCOs, other methods of varying the frequency (such as altering the charging rate of a capacitor by means of a voltage-controlledcurrent source) are used (seefunction generator).
The frequency of aring oscillator is controlled by varying either the supply voltage, the current available to each inverter stage, or the capacitive loading on each stage.
VCOs are used in analog applications such asfrequency modulation andfrequency-shift keying. The functional relationship between the control voltage and the output frequency for a VCO (especially those used atradio frequency) may not be linear, but over small ranges, the relationship is approximately linear, and linear control theory can be used. A voltage-to-frequency converter (VFC) is a special type of VCO designed to be very linear over a wide range of input voltages.
Modeling for VCOs is often not concerned with the amplitude or shape (sinewave, triangle wave, sawtooth) but rather its instantaneous phase. In effect, the focus is not on the time-domain signalA sin(ωt+θ0) but rather the argument of the sine function (the phase). Consequently, modeling is often done in the phase domain.
The instantaneous frequency of a VCO is often modeled as a linear relationship with its instantaneous control voltage. The output phase of the oscillator is the integral of the instantaneous frequency.
For analyzing a control system, theLaplace transforms of the above signals are useful.
Tuning range, tuning gain andphase noise are the important characteristics of a VCO. Generally, low phase noise is preferred in a VCO. Tuning gain and noise present in the control signal affect the phase noise; high noise or high tuning gain implies more phase noise. Other important elements that determine the phase noise are sources offlicker noise (1/f noise) in the circuit,[8] the output power level, and the loadedQ factor of the resonator.[9] (seeLeeson's equation). The low-frequency flicker noise affects the phase noise because the flicker noise isheterodyned to the oscillator output frequency due to the non-linear transfer function of active devices. The effect of flicker noise can be reduced withnegative feedback that linearizes the transfer function (for example,emitter degeneration).
VCOs generally have a lower Q factor compared to similar fixed-frequency oscillators, and so suffer morejitter. 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).
Commonly used VCO circuits are theClapp andColpitts oscillators. The more widely used oscillator of the two is Colpitts and these oscillators are very similar in configuration. Inintegratedradio transmitters andreceivers,cross-coupled LC oscillators are widely used due to their differential output signal, which offers advantages over traditional single-ended topologies such as Clapp and Colpitts oscillators.[10]
Avoltage-controlled crystal oscillator (VCXO) is used for fine adjustment of the operating frequency. The frequency of a voltage-controlled crystal oscillator can be varied a few tens of parts per million (ppm) over a control voltage range of typically 0 to 3 volts, because the high Q factor of the crystals allows frequency control over only a small range of frequencies.
Atemperature-compensated VCXO (TCVCXO) incorporates components that partially correct the dependence on temperature of theresonant frequency of the crystal. A smaller range of voltage control then suffices to stabilize the oscillator frequency in applications wheretemperature varies, such asheat buildup inside atransmitter.
Placing the oscillator in acrystal oven at a constant but higher-than-ambient temperature is another way to stabilize oscillator frequency. High-stability crystal oscillator references often place the crystal in an oven and use a voltage input for fine control.[11] The temperature is selected to be theturnover temperature: the temperature where small changes do not affect the resonance. The control voltage can be used to occasionally adjust the reference frequency to aNIST source. Sophisticated designs may also adjust the control voltage over time to compensate for crystal aging.[citation needed]
Aclock 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 ofphase noise that must be minimised in applications such as radio receivers, transmitters and measuring equipment.
When a wider selection of clock frequencies is needed, the VCXO output can be passed through digital divider circuits to obtain lower frequencies or be fed to aphase-locked loop (PLL). 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 to 96 kHz to an audiodigital-to-analog converter.
Afrequency synthesizer generates precise and adjustable frequencies based on a stable single-frequency clock. Adigitally controlled oscillator based on a frequency synthesizer may serve as a digital alternative to analog voltage-controlled oscillator circuits.
VCOs are used infunction generators,phase-locked loops, includingfrequency synthesizers used in communication equipment and the production ofelectronic music, to generate variable tones insynthesizers.
Function generators are low-frequency oscillators that 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 loops for radio receivers. Phase noise is the most important specification in this application.[citation needed]
Audio-frequency VCOs are used in analog music synthesizers. For these, sweep range, linearity, and distortion are often the most important specifications. Audio-frequency VCOs for use in musical contexts were largely superseded in the 1980s by their digital counterparts,digitally controlled oscillators (DCOs), due to their output stability in the face of temperature changes during operation. Since the 1990s, musical software has become the dominant sound-generating method.
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 signal suitable for transmission over a long distance, since the frequency will not drift or be affected by noise. Oscillators in this application may have sine or square wave outputs.
Where the oscillator drives equipment that may generate radio-frequency interference, adding a varying voltage to its control input, calleddithering,[12][13][14][15][16][17][excessive citations] can disperse the interference spectrum to make it less objectionable (seespread-spectrum clock).
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