Voltage-controlled oscillator

A voltage-controlled oscillator or VCO is an electronicoscillator whose oscillation frequency is controlled bya voltage input. The applied input voltage determinesthe instantaneous oscillation frequency. Consequently,modulating signals applied to control input may causefrequency modulation (FM) or phase modulation (PM).A VCO may also be part of a phase-locked loop.

A microwave (12-18 GHz) Voltage Controlled Oscillator

1 Types of VCOVCOs can be generally categorized into two groups basedon the type of waveform produced: 1) harmonic oscilla-tors, and 2) relaxation oscillators.Linear or harmonic oscillators generate a sinusoidalwaveform. Harmonic oscillators in electronics usuallyconsist of a resonator with an amplier that replaces theresonator losses (to prevent the amplitude from decay-ing) and isolates the resonator from the output (so theload does not aect the resonator). Some examples ofharmonic oscillators are LC-tank oscillators and crystaloscillators. In a voltage-controlled oscillator, a voltageinput controls the resonant frequency. A varactor diodescapacitance is controlled by the voltage across the diode.Consequently, a varactor can be used to change the ca-pacitance (and hence the frequency) of an LC tank. Avaractor can also change (pull) the resonant frequencyof a crystal resonator.Relaxation oscillators can generate a sawtooth or tri-angular waveform. They are commonly used in mono-lithic integrated circuits (ICs). They can provide a widerange of operational frequencies with a minimal numberof external components. Relaxation oscillator VCOs canhave three topologies: 1) grounded-capacitor VCOs, 2)emitter-coupled VCOs, and 3) delay-based ring VCOs.The rst two of these types operate similarly. The timespent in each state depends on the rate of charge or dis-

charge of a capacitor. The delay-based ring VCO op-erates somewhat dierently however. For this type, thegain stages are connected in a ring. The output frequencyis then a function of the delay in each stage.Harmonic oscillator VCOs have these advantages over re-laxation oscillators.

Frequency stability with respect to temperature,noise, and power supply is much better for harmonicoscillator VCOs.

They have good accuracy for frequency control sincethe frequency is controlled by a crystal or tank cir-cuit.

A disadvantage of harmonic oscillator VCOs is that theycannot be easily implemented in monolithic ICs. Relax-ation oscillator VCOs are better suited for this technol-ogy. Relaxation VCOs are also tunable over a wider rangeof frequencies.

2 Control of frequency in VCOs

Voltage-controlled oscillator schematic - audio

A voltage-controlled capacitor is one method of mak-ing an LC oscillator vary its frequency in response to acontrol voltage. Any reverse-biased semiconductor diodedisplays a measure of voltage-dependent capacitance andcan be used to change the frequency of an oscillator byvarying a control voltage applied to the diode. Special-purpose variable capacitance varactor diodes are avail-able with well-characterized wide-ranging values of ca-pacitance. Such devices are very convenient in the man-

1

2 3 VOLTAGE-CONTROLLED CRYSTAL OSCILLATORS

ufacture of voltage-controlled oscillators[note 1] For low-frequency VCOs, other methods of varying the frequency(such as altering the charging rate of a capacitor by meansof a voltage controlled current source) are used. SeeFunction generator.The frequency of a ring oscillator is controlled by vary-ing either the supply voltage, the current available to eachinverter stage, or the capacitive loading on each stage.

3 Voltage-controlled crystal oscil-lators

A voltage-controlled crystal oscillator (VCXO) is used forne adjustment of the operating frequency. The fre-quency of a voltage-controlled crystal oscillator can bevaried a few tens of parts per million (ppm), because thehigh Q factor of the crystals allows pulling over only asmall range of frequencies.There are two reasons for using a VCXO:

To adjust the output frequency to match (or perhapsbe some exact multiple of) an accurate external ref-erence.

Where the oscillator drives equipment that may gen-erate radio-frequency interference, adding a varyingvoltage to its control input can disperse the interfer-ence spectrum to make it less objectionable. Seespread spectrum clock.

A temperature-compensated VCXO (TCVCXO) incor-porates components that partially correct the depen-dence on temperature of the resonant frequency of thecrystal. A smaller range of voltage control then suf-ces to stabilize the oscillator frequency in applicationswhere temperature varies, such as heat buildup inside atransmitter.Placing the oscillator in a temperature-controlled ovenat a constant but higher-than-ambient temperature is an-other way to stabilize oscillator frequency. High stabilitycrystal oscillator references often place the crystal in anoven and use a voltage input for ne control.[1] The tem-perature is selected to be the turnover temperature: thetemperature where small changes do not aect the reso-nance. The control voltage can be used to occasionallyadjust the reference frequency to a NIST source. Sophis-ticated designs may also adjust the control voltage overtime to compensate for crystal aging.

3.1 Phase-domain equationsAnalog applications such as frequency modulation andfrequency-shift keying often need to control an oscillatorfrequency with an input a voltage-controlled oscillator

A 26 MHz TCVCXO.

(VCO). The functional relationship between the controlvoltage and the output frequency may not be linear, butover small ranges, the relationship is approximately lin-ear, and linear control theory can be used.Real VCOs (especially those used at radio frequency)may have non-linear relationship, but linear control the-ory models are still useful. There are devices calledvoltage-to-frequency converters (VFC). These devicesare often designed to be very linear over a wide rangeof input voltages.Modeling for VCOs is often not concerned with the am-plitude or shape (sinewave, triangle wave, sawtooth) butrather its instantaneous phase. In eect, the focus is noton the time-domain signal A sin(t+0) but rather theargument of the sine function (the phase). Consequently,modeling is often done in the phase domain.The instananeous frequency of a VCO is often modeledas a linear relationship with its instaneous control voltage.The output phase of the oscillator is the integral of theinstaneous frequency.

f(t) = f0 +K0 vin(t)

(t) =

Z t1

f() d

f(t) is the instantaneous fre-quency of the oscillator at timet (not the waveform ampli-tude)

3 f0 is the quiescent frequencyof the oscillator (not the wave-form amplitude)

K0 is called the oscillator sen-sitivity, or gain. Its units arehertz per volt.

f(t) is the VCOs frequency (t) is the VCOs output phase vin(t) is the time-domain con-trol input or tuning voltage ofthe VCO

For analyzing a control system, the Laplace transforms ofthe above signals are useful.

F (s) = K0 Vin(s)

(s) =F (s)

s

4 VCO design and circuitsTuning range, tuning gain and phase noise are the im-portant characteristics of a VCO. Generally low phasenoise is preferred in the VCO. The noise present in thecontrol signal and the tuning gain aect the phase noise;high noise or high tuning gain imply more phase noise.Other important elements that determine the phase noiseare the transistors icker noise (1/f noise),[2] the outputpower level, and the loaded Q of the resonator.[3] SeeLeesons equation. The low frequency icker noise af-fects the phase noise because the icker noise is hetero-dyned to the oscillator output frequency due to the ac-tive devices non-linear transfer function. The eect oficker noise can be reduced with negative feedback thatlinearizes the transfer function (for example, emitter de-generation).Leesons expression[4] for single-sideband (SSB) phasenoise in dBc/Hz (decibels relative to output level perHertz) is[5]

L(fm) = 10 log12

f0

2Qlfm

2+1

fcfm

+

1

FkTPs

where f0 is the output frequency, Q is theloaded Q, f is the oset from the output fre-quency (Hz), f is the 1/f corner frequency,F is the noise factor of the amplier, k isBoltzmanns constant, T is absolute tempera-ture in Kelvins, and P is the oscillator outputpower.

Commonly used VCO circuits are the Clapp and Colpittsoscillators. The more widely used oscillator of the two is

Colpitts and these oscillators are very similar in congu-ration.VCOs generally have the lowest Q-factor of the used os-cillators, and so suer 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 theadvantages of having no o-chip components (expensive)or on-chip inductors (low yields on generic CMOS pro-cesses). These oscillators also have larger tuning rangesthan the other kinds, which improves yield and is some-times a feature of the end product (for instance, the dotclock on a graphics card which drives a wide range ofmonitors).

5 Applications

VCOs are used in:

Function generators,

The production of electronicmusic, to generate vari-able tones in synthesizers,

Phase-locked loops,

Frequency synthesizers used in communicationequipment.

Voltage-to-Frequency converters are voltage-controlledoscillators, with a highly linear relation between appliedvoltage and frequency. They are used to convert a slowanalog 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 aected by noise.VCOs may have sine and/or square wave outputs. Func-tion generators are low-frequency oscillators which fea-ture multiple waveforms, typically sine, square, and tri-angle waves. Monolithic function generators are voltage-controlled. Analog phase-locked loops typically containVCOs. High-frequency VCOs are usually used in phase-locked loops for radio receivers. Phase noise is the mostimportant specication for them. Low-frequency VCOsare used in analog music synthesizers. For these, sweeprange, linearity, and distortion are often most importantspecs. Audio-frequency VCOs for use in musical con-texts were largely superseded in the 1980s by their digitalcounterparts, DCOs, due to their output stability in theface of temperature changes during operation. From the1990s on, pure software is the primary sound-generatingmethod, but VCOs have become popular again oftenthanks to their imperfections.

4 10 EXTERNAL LINKS

6 Voltage-controlled crystal oscil-lator as a clock generator

A clock generator is an oscillator that provides a timingsignal to synchronize operations in digital circuits. VCXOclock generators are used in many areas such as digitalTV, modems, transmitters and computers. Design pa-rameters for a VCXO clock generator are tuning voltagerange, center frequency, frequency tuning range and thetiming jitter of the output signal. Jitter is a form of phasenoise that must beminimised in applications such as radioreceivers, transmitters and measuring equipment.The tuning range of a VCXO is typically a few parts permillion over a control voltage range of typically 0 to 3volts. When a wider selection of clock frequencies isneeded the VCXO output can be passed through digi-tal divider circuits to obtain lower frequency(ies) or befed to a PLL (Phase Locked Loop). ICs containing botha VCXO (for external crystal) and a PLL are available.A typical application is to provide clock frequencies in arange from 12 kHz to 96 kHz to an audio digital to analogconverter.

7 See also VFO VCF VCA LFO modular synthesizer Digitally controlled oscillator, DCO Numerically controlled oscillator, NCO Phase-locked loop, PLL

8 Notes[1] A voltage-controlled inductor would be in principle as

useful, but such devices are unsatisfactory at the frequen-cies usually desired.

9 References[1] For example, an HP/Agilent 10811 reference oscillator

[2] Wideband VCO from Herley - General Microwave - Foroptimum performance, the active element used is a sil-icon bipolar transistor. (This is in lieu of GaAs FETswhich typically exhibit 10-20 dB poorer phase noise per-formance)"

[3] Rhea, Randall W. (1997), Oscillator Design & Com-puter Simulation (Second ed.), McGraw-Hill, ISBN 0-07-052415-7

[4] Leeson, D. B. (February 1966), A Simple Model ofFeedback Oscillator Noise Spectrum, Proceedings of theIEEE 54 (2): 329330, doi:10.1109/PROC.1966.4682

[5] Rhea 1997, p. 115

10 External links schematics Designing VCOs and Buers Using the UPA familyof Dual Transistors

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Types of VCOControl of frequency in VCOs Voltage-controlled crystal oscillators Phase-domain equations

VCO design and circuitsApplications Voltage-controlled crystal oscillator as a clock generatorSee also Notes ReferencesExternal linksText and image sources, contributors, and licensesTextImagesContent license