OCXO, Oven Controlled Crystal Oscillator
- the OCXO, oven controlled crystal oscillator or crystal oven is used to provide considerable stability and frequency accuracy improvements over a straightforward crystal oscillator.
Oven Controlled crystal (Xtal) Oscillators, OCXOs, are used in applications where a very high degree of frequency stability is required. Sometimes these oscillators may even be referred to as temperature stabilised crystal oscillators, or just crystal ovens.
While crystal oscillators show a high degree of stability even when the outside temperature is varied over a significant range, for some applications even higher levels of temperature stability are required. In these applications OCXOs may provide the required solution.
As with many other crystal based products, OCXOs are available in a widely variation of packages and package styles. The performance levels and costs also need to be considered as these can vary considerably as well.
Like any physical item, quartz crystals are subject to slight changes as a result of temperature variations. These changes reflect back into the resonant frequency of the crystal causing slight variations. The degree of variation is highly dependent upon the way the crystal is cut during manufacture. The angles of the plane of the blank with reference to the axes of the original crystal determine many of its properties. These include the mode of vibration, the degree of the piezo-electric effect - i.e. its activity, and of course the temperature stability.
A variety of types of crystal cut are used within ovens these days. The AT cut was an obvious type, but as its minimum drift with temperature occurs at around 20°C, this is not the most suitable
Typical frequency / temperature curve for an AT-cut quartz crystal
In the 1970s other cuts including the SC cut was developed. This Stress Compensated cut is less sensitive to mechanical and thermal stress giving it a much more suitable performance for OCXO operation. In addition to this it has a lower temperature turning point, LTTP with a very shallow slope in the range 70 to 85°C where many OCXOs run. The reason that the slope is shallow is that the infection temperature is in the region of 90°C as opposed to 25°C in the case of the AT cut crystal.
More recently another cut, termed the IT cut has been developed. This overcomes issues with the requirement for the LTTP to approach the inflection temperature. To address this issue the IT cut, has an upper temperature turning point in the range of 85 to 105°C. However it is not totally ideal because it does not provide the lower level of mechanical stress sensitivity of the SC cut.
Options for improved frequency stability
Different applications will require different levels of performance. In some cases a crystal oscillator alone may be satisfactory, whereas for other applications a temperature compensated oscillator may be needed. For the highest specification requirements an oven controlled oscillator may be needed.
|Temperature range||Basic crystal oscillator||TCXO||OCXO|
|0C to 70C||±10 ppm||±0.5 ppm||±0.003 ppm|
|-20°C to 70°C||± 25ppm||± 0.5ppm||± 0.02ppm|
|-40°C to 85°C||± 30ppm||± 1ppm||± 0.003ppm|
For applications that need even higher levels of stability, options including rubidium standards or some off-air GPS standards may provide the required levels of accuracy.
Despite this it is still sometimes necessary to ensure a better degree of stability. This can be achieved by placing the crystal in a thermally insulated container with a thermostatically controlled heater. By heating the crystal to a temperature above that which would normally be encountered within the electronic equipment the temperature of the crystal can be maintained at a constant temperature. This results in a far greater degree of temperature stability. Additionally the crystal in the OCXO will be cut to ensure that its temperature stability is optimised for the internal operating temperature.
Commonly the internal temperature for a crystal oven is run at a temperature of 75°C or thereabouts. The temperature needs to be above the highest temperature likely to be encountered, otherwise the temperature control will not work.
The typical specification for an OCXO might be ±5 x 10-8 per degree Celsius (0.05 ppm), whereas a non-oven controlled oscillator may be between 10 and 100 times poorer. As the oscillator assembly will also contain buffering circuitry as well as supply voltage regulation the other characteristics of the oscillator should also be good. Typically it might be expected that frequency stability would be around ±5 x 10-9 (0.005 ppm) per day and ±5 x 10-7 (0.5 ppm) per year and 1 x 10-7 for a 5% change in supply voltage. All of these are far better than would be expected from a simple crystal oscillator.
In order to ensure that the optimum overall accuracy is maintained, combating elements such as ageing of the crystal itself, a periodic calibration of the OCXO may be required. Typical calibration periods may be of the order of six months to a year, but the actual period will depend upon the OCXO itself and the requirements of the application in which it is being used.
OCXO physical considerations
OCXOs are physically much larger than a simple crystal oscillator. Not only do they need to incorporate the crystal oscillator itself, but also the heater, control circuitry and the thermal insulation around the crystal oscillator.
Typically the heater will be run from a different supply to the oscillator. It does not need the same level of regulation, and indeed the oscillator is most likely to have its own regulator to remove any stray noise and RF that may appear on the supply line and thereby degrade the performance of the OCXO.
The supply for the heater in the OCXO may be quite current hungry. Some units may require an Amp or so on warm up. This figure will reduce as the temperature inside the OCXO rises and less heat is needed. As will be imagined the temperature is thermostatically controlled.
These OCXO units are naturally more expensive than crystals on their own, but the performance of an OCXO is considerably enhanced on that of a simple crystal in an unregulated electrical and physical environment.
By Ian Poole
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