page_title: Voltage-controlled oscillator
section_title: Crystal oscillators
hierarchical_section_title: Voltage-controlled oscillator / Design and circuits / Crystal oscillators
caption_attribution_description: English: Temperature compensated voltage controlled crystal oscillator (TCVCXO), 26 MHz
context_page_description: A voltage-controlled oscillator 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 or phase modulation by applying a modulating signal to the control input. A VCO is also an integral part of a phase-locked loop.
  A voltage-to-frequency converter is a special type of VCO designed to be very linear in frequency control over a wide range of input control voltages.
context_section_description: A voltage-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. 
   
  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.
  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. 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.
caption: A 26 MHz TCVCXO
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