Editing Voltage-controlled oscillator

{{Short description|Oscillator with frequency controlled by a voltage input}}
[[File:General Microwave VCO.png|thumb|right|A microwave (12–18{{nbsp}}GHz) voltage-controlled oscillator]]

A '''voltage-controlled oscillator''' ('''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 [[modulation|modulating]] signal to the control input.  A VCO is also an integral part of a [[phase-locked loop]]. VCOs are used in [[synthesizer]]s to generate a [[waveform]] whose [[Pitch (music)|pitch]] can be adjusted by a voltage determined by a [[musical keyboard]] or other input.

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.<ref name="Godse">{{cite book
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 | first1 = Atul P. 
 | last2  = Bakshi
 | first2 = U. A.
 | title  = Linear Integrated Circuits And Applications
 | publisher = Technical Publications
 | date   = 2009
 | pages  = 497
 | url    = https://books.google.com/books?id=vBoS7tXEvK4C&q=%22voltage+to+frequency+converter%22+linear&pg=PA497
 | isbn   = 978-8189411305
 }}</ref><ref name="Drosg">{{cite book
 | last1  = Drosg
 | first1 = Manfred
 | last2  = Steurer
 | first2 = Michael Morten
 | title  = Dealing with Electronics
 | publisher = Walter de Gruyter GmbH
 | date   = 2014
 | pages  = 4.5.3 
 | url    = https://books.google.com/books?id=9RTpBQAAQBAJ&q=%22voltage+to+frequency+converter%22+linear&pg=SA4-PA80
 | isbn   = 978-3110385625
 }}</ref><ref name="Salivahanan">{{cite book
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 | title  = Linear Integrated Circuits
 | publisher = Tata McGraw-Hill Education
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 | url    = https://books.google.com/books?id=rvvMkSM7O84C&q=%22voltage+to+frequency+converter%22+linear&pg=PA515
 | isbn   = 978-0070648180
 }}</ref>

==Types==
VCOs can be generally categorized into two groups based on the type of waveform produced.<ref>{{Cite news|last=Electrical4U|title=Voltage Controlled Oscillator {{!}} VCO {{!}} Electrical4U|url=https://www.electrical4u.com/voltage-controlled-oscillator/|access-date=2021-04-22|newspaper=Electrical4U|language=en-US}}</ref>

* ''Linear'' or ''[[harmonic oscillator]]s'' generate a sinusoidal waveform.  Harmonic oscillators in electronics usually consist of a resonator with an amplifier that replaces the resonator losses (to prevent the amplitude from decaying) and isolates the resonator from the output (so the load does not affect the resonator). Some examples of harmonic oscillators are [[LC oscillator]]s and [[crystal oscillator]]s.
* ''[[Relaxation oscillator]]s'' can generate a sawtooth or triangular waveform. They are commonly used in [[integrated circuit]]s (ICs). They can provide a wide range of operational frequencies with a minimal number of external components.

==Frequency control==
[[File:Voltage controlled oscillator.svg|thumb|350px|right|Schematic of an audio-frequency voltage-controlled oscillator]]

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. A varactor is used to change the capacitance (and hence the frequency) of an LC tank. A varactor can also change loading on a crystal resonator and pull its resonant frequency. 

The same effect occurs with bipolar [[transistor]]s, as described by Donald<ref name="ThomasName">{{Cite web |url=https://ieeexplore.ieee.org/author/37307984700 |title=D. E. Thomas |publisher=IEEE Xplore |access-date=15 January 2025}}</ref> E. Thomas at [[Bell Labs]] in 1954: with a [[tank circuit]] connected to the collector and the modulating audio signal applied between the emitter and the base, a single-transistor FM transmitter is created.<ref name="ThomasTech">{{cite journal |last1= Thomas |first1= D. E. |date= February 1954 |title= Single-Transistor F-M Transmitter |journal= Electronics |volume= 27 |issue= 2 |pages= 130–133 |publisher= McGraw-Hill |url= https://dn720906.ca.archive.org/0/items/sim_electronics_1954-2_27_2/sim_electronics_1954-02_27_2.pdf |access-date= 14 January 2025}}</ref> Thomas worked with a [[point-contact transistor]], but the effect also works in [[junction transistor]]s; applications include [[wireless microphone]]s such as that patented by [[Raymond A. Litke]] in 1964.<ref name="LitkePatent">{{cite patent| country= US| number= 3134074| invent1= Ray A. Litke| status= patent| title= Microphone transmitter having a lavalier type antenna| gdate= 1964-05-19| fdate= 1961-05-08| assign1= Vega Electronics Corporation| url= https://patentimages.storage.googleapis.com/89/c2/c8/c2eb65db11efb9/US3134074.pdf}}</ref>

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, the current available to each inverter stage, or the capacitive loading on each stage.

===Phase-domain equations===
<!-- this is still messed up -->
VCOs are used in analog applications such as [[frequency modulation]] and [[frequency-shift keying]]. The functional relationship between the control voltage and the output frequency for a VCO (especially those used at [[radio 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 signal {{math|''A'' sin(''&omega;t''+''&theta;''<sub>0</sub>)}} 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.
:<math>\begin{align}
       f(t) &= f_0 + K_0 \cdot \ v_\text{in}(t) \\
  \theta(t) &= \int_{-\infty}^t f(\tau)\,d\tau \\
\end{align}</math>
::* <math>f(t) </math> is the instantaneous frequency of the oscillator at time {{mvar|t}} (not the waveform amplitude)
::* <math>f_0 </math> is the quiescent frequency of the oscillator (not the waveform amplitude)
::* <math>K_0 </math> is called the oscillator sensitivity, or gain. Its units are hertz per volt.
::* <math>f(\tau) </math> is the VCO's frequency
::* <math>\theta(t) </math> is the VCO's output phase
::* <math>v_\text{in}(t) </math> is the time-domain control input or tuning voltage of the VCO

For analyzing a control system, the [[Laplace transform]]s of the above signals are useful.<!-- missing impulse; scaling wrong; need to move constant -->
:<math>\begin{align}
       F(s) &= K_0 \cdot \ V_\text{in}(s) \\
  \Theta(s) &= {F(s) \over s} \\
\end{align} </math>

==Design and circuits==
Tuning range, tuning gain and [[phase 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 imply more phase noise. Other important elements that determine the phase noise are sources of [[flicker noise]] (1/''f'' noise) in the circuit,<ref>[http://www.herley.com/index.cfm?act=product&prd=481 Wideband VCO] from [[Herley Industries|Herley - General Microwave]] - "For optimum performance, the active element used is a silicon bipolar transistor. (This is in lieu of GaAs FETs which typically exhibit 10-20 dB poorer phase noise performance)" {{webarchive |url=https://web.archive.org/web/20120308060851/http://www.herley.com/index.cfm?act=product&prd=481 |date=8 March 2012}}</ref> the output power level, and the loaded [[Q factor]] of the resonator.<ref>{{Citation |last=Rhea |first=Randall W. |title=Oscillator Design & Computer Simulation |edition=Second |year=1997 |publisher=McGraw-Hill |isbn=0-07-052415-7 }}</ref> (see [[Leeson's equation]]). The low frequency flicker noise affects the phase noise because the flicker noise is [[heterodyne]]d to the oscillator output frequency due to the non-linear transfer function of active devices. The effect of flicker noise can be reduced with [[negative feedback]] that linearizes the transfer function (for example, [[common emitter#Emitter degeneration|emitter degeneration]]).

VCOs generally have lower Q factor compared to similar fixed-frequency oscillators, and so suffer more [[jitter]].  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).

=== LC oscillators ===
Commonly used VCO circuits are the [[Clapp oscillator|Clapp]] and [[Colpitts oscillator|Colpitts]] oscillators. The more widely used oscillator of the two is Colpitts and these oscillators are very similar in configuration.

=== Crystal oscillators ===
[[File:Skymaster DT 500 - TLSI T73227-91730.jpg|thumb|27{{nbsp}}MHz VCXO clock generator IC (TLSI T73227), used in a [[DVB-T]] set-top box.]]

A '''{{visible anchor|voltage-controlled crystal oscillator|VCXO}}''' ('''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.

[[File:SMD Cystal Oscillator TCXO.png|thumb|A 26{{nbsp}}MHz TCVCXO]]

A '''{{visible anchor|temperature-compensated VCXO|TCVCXO}}''' ('''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]].

Placing the oscillator in a [[crystal 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.<ref>For example, an HP/Agilent 10811 reference oscillator</ref> The temperature is selected to be the ''turnover temperature'': the temperature where small changes do not affect the resonance. The control voltage can be used to occasionally adjust the reference frequency to a [[NIST]] source. Sophisticated designs may also adjust the control voltage over time to compensate for crystal aging.<!-- quadratic curve fit of frequency drift. Whose appnote? -->{{citation needed|date=April 2012}}

===Clock generators===
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.

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 a [[phase-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&nbsp;kHz to 96&nbsp;kHz to an audio [[digital-to-analog converter]].

=== Frequency synthesizers===
A [[frequency synthesizer]] generates precise and adjustable frequencies based on a stable single-frequency clock. A [[digitally controlled oscillator]] based on a frequency synthesizer may serve as a digital alternative to analog voltage controlled oscillator circuits.

== Applications ==
[[File:Korg Monologue Synthesizer.jpg|thumb|The [[Korg Monologue]] is a monophonic synthesizer with two VCOs.]]
VCOs are used in [[function generator]]s, [[phase-locked loop]]s including [[frequency synthesizer]]s used in communication equipment and the production of [[electronic music]], to generate variable tones in [[synthesizer]]s.

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 loops for radio receivers. Phase noise is the most important specification in this application.{{citation needed|date=December 2017}}

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 oscillator]]s (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, called [[wikt:dither|dithering]],<ref name="dither5">{{cite web |title=Frequency Modulation of System Clocksfor EMI Reduction |url=https://www.hpl.hp.com/hpjournal/97aug/aug97a13.pdf |website=hpl.hp.com |publisher=HP |access-date=23 January 2020}}</ref><ref name="dither4">{{cite web |title=EMI Reduction by Spread-Spectrum Frequency Dithering |url=https://incompliancemag.com/article/emi-reduction-by-spread-spectrum-frequency-dithering/ |website=incompliancemag.com |publisher=Same Page Publishing |access-date=23 January 2020}}</ref><ref name="dither3">{{cite web |title=Oscillator – spread-spectrum resistor-programmable |url=https://www.planetanalog.com/oscillator-spread-spectrum-resistor-programmable/# |website=www.planetanalog.com |publisher=Planet Analog |access-date=23 January 2020}}</ref><ref name="dither2">{{cite journal |title=Frequency Dithering With the UCC28950 and TLV3201 |journal=TI Application Report |date=May 2012 |volume=SLUA646 |publisher=TI |location=frequency-dithering-with-the-ucc28950-and-tlv3201-1339689710.pdf }}</ref><ref name="dither1">{{cite web |last1=Bell |first1=Bob |title=Dither a power converter's operatingfrequency to reduce peak emissions |url=https://m.eet.com/media/1130732/14099-101305di.pdf |website=m.eetcom |publisher=EE Times |access-date=23 January 2020}}</ref><ref name="dither0">{{cite web |title=PFC Pre-Regulator Frequency Dithering Circuit |url=http://www.ti.com/lit/an/slua424a/slua424a.pdf |website=www.ti.com |publisher=TI |access-date=23 January 2020}}</ref>{{Excessive citations inline|date=November 2022}} can disperse the interference spectrum to make it less objectionable (see [[spread spectrum clock]]).<!--[[User:Kvng/RTH]]-->

== See also ==
* [[Low-frequency oscillation]] (LFO)
* [[Modular synthesizer]]
* [[Numerically-controlled oscillator]] (NCO)
* [[Variable-frequency oscillator]] (VFO)
* [[Variable-gain amplifier]] 
* [[Voltage-controlled filter]] (VCF)

==References==
{{Reflist}}

==External links==
*{{cite web |url=http://my.integritynet.com.au/purdic/voltage-controlled-oscillators.htm |archive-url=https://web.archive.org/web/20190104043804/http://my.integritynet.com.au/purdic/voltage-controlled-oscillators.htm |archive-date=2019-01-04 |title=Design of V.C.O.'s |website=Ian Purdie's Amateur Radio Tutorial Pages |access-date=2018-01-28}}
*[http://www.cel.com/pdf/appnotes/an1034.pdf Designing VCOs and Buffers Using the UPA family of Dual Transistors]

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[[Category:Electronic oscillators]]
[[Category:Synthesizer electronics]]
[[Category:Radio electronics]]
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