VCXO

VCXO is not something that the average hobbyist will run into very often, but it is good to know about it nonetheless in case you do. There are only certain conditions in which VCXO is used.

A voltage-controlled crystal oscillator (VCXO) is used when the frequency of operation needs to be adjusted by a very small amount. VCXO devices are used when exact frequency or phase of the oscillator is critical. They can also be used for applying a varying voltage to the control input of the oscillator and they can be used to disperse radio-frequency interference over a range of frequencies to make it less objectionable.

For the most part, the usable frequency of a voltage-controlled crystal oscillator can only be varied by a few tens of parts per million (ppm), because the high Q factor of the crystals allows only a small "pulling" range of frequencies to be produced. This tiny change is what makes them so well suited for precision work.

Another type of special VCXO is the temperature-compensated VCXO (TCVCXO). This type of device is normally used so that temperature does not alter the piezoelectric effect created by the crystal. Increased temperatures can cause frequency drift due to the heat buildup inside the transmitter.

In general, a VCXO is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. A common use of this application can be found in time keeping in a quartz wristwatch, or in providing a stable clock signal for digital integrated circuits. It is also used to stabilize frequencies for radio transmitters.

If the basic VCXO set up also includes an amplifier and feedback, it can be an especially accurate form of electronic oscillator. In this type of application, the crystal used is sometimes called a "timing crystal". On schematic diagrams a crystal is labeled with the letter Y.

The crystal oscillator circuit sustains oscillation by taking a voltage signal from the quartz resonator, amplifying it, and feeding it back to the resonator. The rate of expansion and contraction of the quartz is the resonant frequency, and is determined by the cut and size of the crystal.

It should be noted that one of the most important traits of VCXO quartz crystal oscillators is that they can exhibit very low phase noise. What this means is that the signal they produce is a pure tone. This pure tone makes them particularly useful in telecommunications where stable signals are mandatory as well as in scientific equipment where very precise time references are needed.

Do keep in mind that environmental changes of temperature, humidity, pressure, and vibration can change the resonant frequency of a VCXO device.