TCXO performance
When considering a TCXO for an electronic design, it is necessary to consider the major specifications and parameters for the unit.
Typical surface mount technology TCXO
Some of the main performance figures are summarised below:
TCXO stability performance: The TCXO temperature performance is better than that of a normal crystal oscillator. Figures are normally quoted in terms of parts per million, PPM over a given temperature range. Typically figures of between 10 and 40 times improvement can often be seen.
Typical figures are given in the table above for the different temperature ranges. Figures of better than ±1.5 ppm over a 0 to 70°C temperature range are difficult to achieve as they then fall into a high precision category where costs increase significantly.
Power dissipation: 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 stabilise. This may be of the order of 100 ms, or possibly longer, dependent upon the design.
TCXO package: 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.
Package sizes such as 5x3.2x1.5 mm or 5x3.5x1 mm are widely used for TCXOs and smaller packages available if required.
Output format and level: It is important to ensure that the output of the TCXO is suitable for the particular electronic design being undertaken.
The output format for the TCXO is important. With many TCXOs being used for driving digital circuits, most of the small oscillator packages produce what is termed a clipped sine wave. This is suitable for driving a logic circuit, although in many cases it is wise to put it through a logic buffer to ensure it is sufficiently square. Often the output is an open collector circuit. If a sine wave output is required, then this must be chosen at the outset and it will limit the choice available. Also for other RF applications, a sine way may be required.
The level is also important as it should be sufficient to drive the following stage in the electronic circuit design.
Power requirements: The actual power requirements will depend upon the particular device. Many operate from supplies of 3 V, and may draw as little as 2 mA, although this will depend upon the general type, the manufacturer and the particular device chosen. It is obviously necessary to ensure that the power requirements can be met by the circuit.
In circuits that require a particularly clean output in terms of phase noise, etc, it is worth checking on the rejection of power supply noise on the output of the oscillator.
