What's the Difference Between OCXOs and TCXOs?

Quartz crystals such as these are cut to ensure that temperature stability is optimized for the OCXO&#;s internal operating temperature.

If you want to learn more, please visit our website.

The Power to Control Temperature

It&#;s still sometimes necessary to ensure an even 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 crystal&#;s temperature 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 optimized for the internal operating temperature (see figure).

The internal temperature for a crystal oven is commonly run at about 75°C. It needs to be above the highest temperature likely to be encountered, or else the temperature control will not work.

The typical specification for an OCXO might be ±5 × 10&#;8 per degree Celsius (0.05 ppm). A non-oven-controlled oscillator, on the other hand, may be between 10 and 100 times poorer.

To ensure that the optimum overall accuracy is maintained, combating elements such as aging of the crystal itself may be required as well as a periodic calibration of the OCXO. Typical calibration periods may be on the order of six months to a year, but the actual period will depend on the OCXO itself and the requirements of the application in which it is being used.

More Physical Abilities of an OCXO

OCXOs are physically much larger than a simple crystal oscillator. This is because they:

• Incorporate the crystal oscillator itself
• Contain a heater
• Contain control circuitry
• House thermal insulation around the crystal oscillator

Typically, the heater will run from a different supply to the oscillator. The supply for the OCXO&#;s heater may be quite current-hungry. Some units may require an amp or so upon warm-up. This figure will reduce as the temperature inside the OCXO rises and less heat is needed. As you might imagine, the temperature is thermostatically controlled.

Temperature-Compensated Crystal Oscillators (TCXOs)

A standard TCXO has quite a few performance qualities and can even completely solve the two major problems with quartz crystals. Here are some of the most common performance characteristics of TCXOs:

TCXO PPM performance: The TCXO temperature performance is better than that of a normal crystal oscillator. Figures of between 10 and 40 times improvement can often be seen. Figures of better than ±1.5 ppm over a 0 to 70°C temperature range are difficult to achieve, though. That&#;s because they 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 due to the additional circuitry required. In addition, the cost is greater. One should also remember that it takes a short while after startup for the oscillator to stabilize (and hopefully stay stabilized). This may be on the order of 100 ms, or possibly longer, depending on the design.

TCXO package: TCXOs come in a variety of packages, depending on the way they have been designed and the requirements of the end user. The most common form of construction is to build the circuit on a small printed circuit board (PCB) that can be housed in a plated metal package. This then becomes suitable for mounting onto the main circuit board of the overall equipment. Since 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 5 × 3.2 × 1.5 mm or 5 × 3.5 × 1 mm are widely used for TCXOs; smaller packages are available if required.

Output format and level: With many TCXOs being used to drive 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&#;s wise to put it through a logic buffer to ensure it&#;s sufficiently square. Often the output is an open collector circuit. If a sine wave output is required, this must be chosen at the outset; it will limit the choice available.

Link to Huixun

Power requirements: Actual power requirements will be predicated on the device. Many operate from supplies of 3 V and may draw as little as 2 mA, although this will depend on the general type, the manufacturer, and the particular device chosen.

4 Common Types of TCXOs

Although TCXOs are normally referred to in this manner, occasionally more detailed descriptions are used. Consequently, a variety of techniques can be used to provide the temperature compensation.

ADTCXO: This is an analog digital TCXO, widely used in cell phones. ADTCXOs leverage analog technology to provide temperature correction to the oscillator. It has the advantage that changes take place slowly and no phase jumps are experienced, as is the case with some all-digital types.

DTCXO: As you probably surmised, this is a digital TCXO. A DTCXO begins with a temperature sensor. Logic and mathematical functions use digital circuitry along with a lookup table. The resulting digital correction figure is converted to an analog signal using a digital-to-analog 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 deliver 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 OCXOs. As such, they offer an ideal solution for many portable units requiring a reasonably accurate source.

To read more on common misconceptions about crystal oscillator stability, click here.

The oscillation frequency of a crystal changes as temperature varies. Both TCXOs (Temperature Compensated Crystal Oscillators) and OCXOs (Oven Controlled Crystal Oscillators) are crystal oscillators whose oscillation frequency does not vary as much at high temperatures or when the temperature fluctuates.

Design: A TCXO uses a compensation circuit to counter/compensate for the change in output frequency due to temperature variation.

In an OCXO, the crystal oscillator is placed in an oven that is pre-heated to a higher temperature. This way an external temperature variation will not impact the output frequency of the oscillator. Due to the fact that OCXO's use an oven, they usually bulky and require some time to heat the oven before providing the frequency stability vs temperature variation. They also have higher power requirements as compared to TCXO's as the oven requires a supply voltage to maintain a higher temperature.

Size: Due to the fact that OCXO's use a pre-heated oven, they are usually bulky. TCXOs use an electronic circuit to compensate for the frequency variation as a result of temperature, the electronic circuits are much smaller than the oven, thus TCXO's are usually much smaller in size than OCXO's.

Power Consumption: OCXO's are more expensive than TCXO's due to the additional components - Oven and Supporting Circuitary required to pre-heat the oven.

Cost: The cost of an OCXO is significantly higher than a TCXO as the OCXO has more components - mainly the oven and circuitry required to heat the oven.

Difference Between TCXO and OCXO

Use our parametric search tool to find TCXOs and OCXOs from the leading manufacturers.

Contact us to discuss your requirements of ocxo . Our experienced sales team can help you identify the options that best suit your needs.