Voltage Controlled Oscillators

Voltage controlled oscillators are crystal oscillators in which oscillating frequency changes with the amount of voltage that is applied. When AC or radio signals are given to the VCXO, the resulting output is called pulse modulation. Voltage controlled oscillators are used in radio transmitters to modulate frequency and in electronic pianos to produce different tones.

A crystal oscillator is an in electronic circuit which uses the mechanical resonance of a physical crystal of piezoelectric material along with an amplifier and feedback to create an electrical signal which has a very precise and accurate frequency. A crystal oscillator is a very accurate version of an electronic oscillator. This frequency is used to keep track of time such as in quartz wristwatches, to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters. Crystal oscillators are a popular and common source of time and frequency signals. The crystal that is used is often called timing crystal.

Voltage-controlled crystal oscillators are often used when the frequency of operation needs to be adjusted by a relatively small amount or when the exact frequency or phase of the oscillator is critical, or, by applying a varying voltage to the control input of the oscillator, to disperse radio-frequency interference over a range of frequencies to make it less objectionable. The frequency of voltage-controlled crystal oscillators can usually only be varied by a few tens of parts per million (ppm), because the high Q factor of the crystals only enables a small pulling range of frequencies to be produced.

VCXOs come in a variety of frequency ranges, from a few megahertz to gigahertz values. Important considerations when selecting voltage-controlled oscillators are:

  • Linearity: which is the smoothness of the change in frequency when the voltage is changed
  • Rise and fall times: for square waves that measures the trueness of the edges
  • Phase noise: the disruptions caused due to noise
  • Modulation: which allows the VCXO to have a time controlled voltage 

Other factors to be considered are the harmonic distortion levels, hysteresis (which is the variation in frequency at 25 deg C), the tuning voltages, spurious decibels, the pushing and pulling frequencies, input voltages and the operating temperatures range.

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