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Voltage controlled crystal oscillators, VCXOs find uses in many RF circuits where the stability of a crystal oscillator or xtal oscillator is needed, but with the ability to pull or adjust them by a small amount.
Using a VCXO enables high levels of stability combined with low levels of phase noise to be achieved. Often VCXOs are used where a small programmable adjustment of a signal may be required, for example as the reference signal within an electronic system Here the VCXO can be programmed using a voltage derived from a digital signal.
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.
VCXOs are used in many applications. Here are a few:
- Temperature controlled crystal oscillators : VCXOs are used in TCXOs - temperature controlled crystal oscillators, 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 of a VCXO is still not as good as a full oven controlled crystal oscillator, it is able to provide a level of performance better than that of a single uncompensated crystal oscillator and for much less than the cost of a full oven controlled oscillator.
- Narrow band PLLs: In another application, VCXOs can be used in narrow band phase locked loops where only a small amount of frequency variation is required. Here they are able to provide very good levels of phase noise. They may be used for signal reconstitution in this role.
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.
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.
- 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.
- Output: VCXOs are required for a number of applications. Some may be used 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.
- Operating temperature range: Most voltage controlled crystal oscillators 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.
- 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.
- 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.
- Package: It is necessary to specify the package as voltage controlled crystal oscillators 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.
VCXOs are often used within RF circuits. Voltage controlled crystal oscillators can be bought as small modules from many crystal resonator and crystal oscillator manufacturers, and typically a wide range is available. Custom designs can also be manufactured by most suppliers as well. Also, the VCXOs are normally low cost and provide an excellent degree of performance.
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