TCXO Performance Qualities
A standard TCXO has quite a few performance qualities. Some of the most common performance figures are summarized below:
- TCXO PPM performance: The TCXO temperature performance is better than that of a normal crystal oscillator. 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 stabilize. 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.
Note: 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: 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.
- Power requirements: The actual power requirements will depend upon the 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.
4 Common Types of TCXOs
Although temperature compensated crystal oscillators are normally referred to in this manner, occasionally more detailed descriptions are used. This has resulted in the variety of techniques that can be used to provide the temperature compensation.
- ADTCXO: This is an Analogue Digital TCXO. This form of TCXO has been widely used in cell phones. This uses analogue technology to provide temperature correction to the oscillator. It has the advantage that changes take place slowly and no phase jumps are experienced as occurs with some all-digital types.
- DTCXO: As may be guessed, this is a digital TCXO. It uses a temperature sensor and then logic and mathematical functions use digital circuitry along with a look up table. The resulting digital correction figure is converted to an analogue signal using a digital to analogue converter, DAC.
- DCXO: This is a form of oscillator where any correction is calculated by the host processor within the equipment. In this way, the TCXO is not a separate entity, but the processing is incorporated within that of the overall equipment. This can help save costs in some instances.
- MCXO: This form of TCXO uses a microprocessor to provide a considerably increased level of processing to provide more accurate compensation under a variety of circumstances. While performance is a little better, costs are above those of the other forms.
TCXOs are widely used where accurate frequency sources are needed. They are less expensive and smaller than oven controlled crystal oscillators, as such they offer an ideal solution for many portable units requiring a reasonably accurate source.
By the way! With all this temperature talk, have you ever wondered if a crystal oscillator can operate outside its specified temperature range? Check out our blog post to find out!