VCXO, Voltage Controlled Crystal Oscillator

- an overview giving information about the basics of voltage controlled crystal oscillator, VCXO, which is used for generating controllable, or slightly variable but stable frequency reference signals.

Voltage controlled crystal oscillators are used in a number of applications where a stable frequency source is required, but with the additional capability of fine tuning it using an electronic voltage. By using a crystal oscillator as the basis of the circuit, high levels of frequency stability and low levels of phase noise can be maintained while still being able to control the frequency over a small range.

VCXO circuit

The basic circuit for an VCXO comprises a standard crystal oscillator but with an electronic means of tuning or "pulling" the frequency slightly. This is almost invariably achieved using varactor or varicap diodes. In most VCXOs a pair of back to back diodes are placed across the crystal. A reverse bias is applied to anodes of the diodes which then act as a variable capacitor across the crystal. In most cases a Colpitts oscillator circuit is used.

The amount by which the crystal frequency can be pulled depends upon a variety of factors including the level of capacitance applied, the circuit conditions themselves and the crystal. However the frequency cannot be pulled too far, because the activity of the crystal reduces as the level of capacitance across the crystal increases. If it is necessary for the VCXO to be pulled over a large range, then an inductor can be incorporated into the circuit.

VCXO performance

Using this approach, VCXO figures of frequency variation of around 35 to 50 ppm/volt are reasonably easy to achieve and VCXOs with these figures are quite common.

Naturally the fact that the frequency of the VCXO can be pulled reduces the overall performance of the oscillator circuit. The phase noise performance of the oscillator is degraded because the effective Q of the resonator is considerably reduced. Additionally the frequency stability is not as good.

One of the major problems with VCXOs is that of temperature drift. As this varies over the voltage control range, it cannot be optimised for all levels of control voltage, the final design being a compromise. This when used without other forms of temperature compensation they may drift more than other forms of crystal oscillator.

VCXO applications

VCXOs are used in many applications. They are used in TCXOs where the temperature compensation voltage is applied to a control terminal of the VCXO. In this way the drift can be considerably reduced, although the performance is still not as good as a full oven controlled crystal oscillator.

In another application, VCXOs are often found in narrow band phase locked loops where only a small amount of frequency variation is required.