What Is a Voltage-Controlled Oscillator?

Article Details
  • Written By: Joe Williams
  • Edited By: A. Joseph
  • Last Modified Date: 22 September 2016
  • Copyright Protected:
    Conjecture Corporation
  • Print this Article
Free Widgets for your Site/Blog
Built at the request of Dag Hammarskjöld, the United Nations Headquarters has a meditation room dedicated to silence.   more...

October 1 ,  1890 :  Yosemite National Park was established.  more...

A voltage-controlled oscillator (VCO) uses an input voltage to determine its oscillation frequency. This can include the input of direct current (DC) with a fixed voltage to produce an oscillation with a fixed frequency. A voltage-controlled oscillator can also receive a changing voltage to produce an oscillation with a modulated frequency. Some oscillators of this type have a digital pulse output that can produce pulses with a modulated width. A VCO might be classified as a harmonic oscillator or relaxation oscillator, depending on the type of waveform it produces.

Harmonic oscillators produce waveforms with sinusoidal shapes. This type of voltage-controlled oscillator must include an amplifier to feed the signal back to the source of the input voltage, which creates a resonant frequency that produces a positive gain around the feedback loop and creates the sinusoidal waveform. A harmonic oscillator might also include a varactor diode that provides part of the capacitance for the resonant circuit. The oscillation frequency can then be varied according to the diode’s voltage input.

A relaxation oscillator produces a waveform with a triangular shape. This type of voltage-controlled oscillator contains three subtypes, including an emitter-coupled oscillator, a grounded-capacitor oscillator and a delay-based ring oscillator. The amount of time needed to charge the capacitor determines the oscillation frequency for the emitter-coupled oscillator and grounded-capacitor oscillator. The oscillation frequency of the delay-coupled oscillator depends on the amount of time it takes for the oscillator to attain each increase in gain.


Harmonic voltage-controlled oscillators have greater frequency stability than relaxation VCOs over changes in noise, power supply and temperature. The frequency control is also better in a harmonic VCO, since this type uses a separate circuit to control the frequency. The primary advantage of a relaxation VCO is its ability to be tuned over a wider frequency range than the harmonic type. A relaxation VCO also is better suited to use in integrated circuits.

A voltage-controlled oscillator frequently uses a voltage-controlled capacitor to control the output frequency. This type of capacitor contains a semiconductor diode with a capacitance that depends on its voltage. The oscillator’s output frequency can then be controlled by modifying the diode’s input voltage. A voltage-controlled capacitor is a convenient method of controlling the output frequency of a high-frequency VCO because voltage-controlled capacitors are readily available in a wide range of capacitances. A low-frequency VCO typically uses a voltage-controlled source of electrical current to vary the output frequency.


You might also Like


Discuss this Article

Post your comments

exception 'Exception' with message 'error writing captcha: Duplicate entry '2147483647' for key 'PRIMARY'' in /ssd/www/wisegeek/public_html/_core/classes/public/Captcha.php:44
Stack trace:
#0 /ssd/www/wisegeek/public_html/_core/controls/public/ControlDiscussionPostBox.php(324): Captcha->createCaptcha()
#1 /ssd/www/wisegeek/public_html/framework/classes/Control.php(104): ControlDiscussionPostBox->preRender(false)
#2 /ssd/www/wisegeek/public_html/framework/classes/Control.php(149): Control->render()
#3 /ssd/www/wisegeek/public_html/tpl/default-nocustom-lu/pages/public/article/article.htm(526): Control->__toString()
#4 /ssd/www/wisegeek/public_html/framework/classes/Control.php(300): require('/ssd/www/wisege...')
#5 /ssd/www/wisegeek/public_html/framework/classes/Control.php(309): Control->requireTpl('pages/public/ar...', Object(PageArticleCom), true)
#6 /ssd/www/wisegeek/public_html/framework/classes/Control.php(131): Control->renderTpl('pages/public/ar...', Object(PageArticleCom))
#7 /ssd/www/wisegeek/public_html/framework/classes/FormDataControl.php(87): Control->renderTemplate()
#8 /ssd/www/wisegeek/public_html/framework/classes/Control.php(109): FormDataControl->renderTemplate()
#9 /ssd/www/wisegeek/public_html/framework/classes/ScriptPage.php(50): Control->render(false)
#10 /ssd/www/wisegeek/public_html/framework/classes/Control.php(149): ScriptPage->render()
#11 /ssd/www/wisegeek/public_html/framework/classes/Page.php(97): Control->__toString()
#12 /ssd/www/wisegeek/public_html/_core/classes/public/PublicFrontController.php(443): Page->processRequest()
#13 /ssd/www/wisegeek/public_html/_core/classes/public/PublicFrontController.php(7): PublicFrontController->renderPage()
#14 /ssd/www/wisegeek/public_html/index.php(11): PublicFrontController::run()
#15 {main}