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.

Specifying VCXOs

many VCXOs are ordered as modules from a specialist supplier of these items. It is necessary to specify them correctly to obtain the required product.

When specifying a VCXO the following parameters are normally needed:

  • Frequency:   This is normally specified in MHz for frequencies over 1.0MHz and in kHz below this. It also needs to be specified to the correct number of decimal places to enable the manufacturer to be able to determine the required frequency needed. Refer to the manufacturers details for the correct number - typically six or seven significant figures.
  • Output:   VCXOs are required for a number of applications. Some may be sued to drive different forms of logic, whereas others may be required for analogue applications. The output requirements are an important element of the overall specification.
  • Frequency stability:   Even though VCXOs are variable, the basic frequency stability still needs to be specified. Generally this is done for room temperature, 20°C and with the voltage control point set to its centre value. The frequency stability is taken for operation over the operating temperature range. The value is specified in terms of parts per million, ppm. Standard stability specifications are typically ±25ppm, ±50ppm and ±100ppm.
  • Supply voltage:   It is necessary to specify the supply voltage to ensure that it operates within the unit for which it is intended. Ideally it should be able to operate outside the expected tolerance of the supply so that any slight mismatch does not cause an issue.
  • Operating temperature range:   Most units have a temperature operating range over which they will function. Accordingly it is necessary to specify a range for the VCXO. There are three main ranges:

    • 0 - 70° C - Often referred to as a commercial temperature range. It is normally satisfactory for operation within office or laboratory environments.
    • -10 - + 70°C - Industrial range which is needed where temperature ranges may fluctuate more widely.
    • -40 - +85°C - this is nearing the military range (normally -40 to +125°C) and is required where much larger temperature ranges are needed. For example for equipment that may be used externally.
    It should be remembered that there is often a temperature rise within an item of equipment and this means that components will be operating well above the ambient external temperature. This must be taken into consideration when determining what temperature range may be needed.
  • Pullability:   The pullability is the frequency range over which the VCXO can be pulled for a given change in control voltage. It is specified in terms of parts per million, ppm, for a given voltage. As is expected, large pullability figures give a larger tuning range, but VCXOs will smaller levels of pullability offer greater stability and lower phase noise.
  • Package:   It is necessary to specify the package as VCXOs come in many forms and various options may be available from through hole mount to surface mount. Also options such as tape and reel are often available for large scale manufacture using automated manufacturing machinery.

By Ian Poole

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