TCXO, Temperature Compensated Crystal Oscillator

- an overview of the TCXO, used for providing a much higher levels of temperature stability than are possible with a normal crystal oscillator

The temperature controlled xtal or crystal oscillator, TCXO, is a form of crystal oscillator used where a high precision frequency source is required within a small space and at reasonable cost. By compensating within the oscillator for temperature changes, it is possible to considerably improve on the basic performance of the oscillator.

Effect of temperature

Although crystal oscillators offer a highly stable form of oscillator, they are nevertheless affected by temperature. The cut of the actual crystal element from the overall grown crystal can help to minimize the effects of temperature, but they are still affected to some degree. For a crystal cut known as the AT cut, the drift with temperature can be minimized around normal ambient temperature, but the rate of drift will rise above and below this.

The effects o temperature are, to a large degree, repeatable and definable. Therefore it is possible to compensate for many of the effects using a temperature compensated crystal oscillator, TCXO.

TCXO solution

A TCXO adjusts the frequency of the oscillator to compensate for the changes that will occur as a result of temperature changes. To achieve this, the main element within a TCXO is a Voltage Controlled Crystal Oscillator (VCXO). This is connected to a circuit that senses the temperature and applies a small correction voltage to the oscillator.

The temperature sensing and compensating network may take a variety of forms these days. The traditional method was to use a thermistor network. More recently digital techniques such as DSP have been used to enable highly accurate compensation to be achieved.

The problem with applying the temperature compensation in a TCXO is that the temperature coefficient of the crystal changes with temperature, and it is not linear. Accordingly the design of the compensation network is complicated.

Additionally TCXOs normally have an external adjustment to enable the frequency to be reset periodically. This enables the effects of the ageing of the crystal to be removed. The period between calibration adjustments will depend upon the accuracy required, but may typically be six months or a year. Shorter periods may be sued if very high levels of accuracy are required.

TCXO performance

The performance of a TCXO is generally considerably better than that of a normal crystal oscillator. Figures within the range 1 to 5 ppm are often achieved. However it should be mentioned that some of this figure comes from a frequency / temperature hysteresis, dependent upon whether the temperature was increasing or decreasing.

The power dissipation of a TCXO will be greater than an ordinary oscillator in view of the additional circuitry required. Additionally the cost is greater. It should also be remembered that it will take a short while after start up for the oscillator to stabilize. This may be of the order of 100 ms, or possibly longer, dependent upon the design.

TCXO packages

TCXOs can be supplied in a variety of packages dependent upon the way they have been designed and the requirements of the end user. The most common form of construction is to construct the circuit on a small printed circuit board that can be house in a plat metal package. This is then suitable for mounting onto the main circuit board of the overall equipment. As the crystal itself is sealed, this means that sealing of the overall TCXO package is not critical, or even required for most applications.

When TCXOs utilize ASIC technology this often enables much smaller packages to be used. These may even be in surface mount packages that are far more suitable for today's manufacturing techniques

Summary

TCXOs are widely used where accurate frequency sources are needed. They are less expensive and smaller than oven controlled oscillators, as such they offer an ideal solution for many portable units requiring a reasonably accurate source.