What is the difference between OCXO and TCXO?

Quartz crystals are specifically shaped to maximize temperature stability within the internal operating range of OCXOs.

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The Power to Control Temperature

For heightened stability, quartz crystals can be placed in an insulated chamber equipped with a thermostatically controlled heater. This method provides improved temperature control by maintaining the crystal's temperature above typical operational levels, thus ensuring steady performance. The OCXO crystal is cut to enhance stability at this regulated temperature.

Typically, the internal temperature of a crystal in an oven is set around 75°C, ensuring it exceeds any expected maximum temperature fluctuations to facilitate effective temperature management.

While an OCXO might boast specifications of ±5 x 10^-8 per degree Celsius (0.05 ppm), a standard oscillator lacking an oven control mechanism may vary significantly worse, performing anywhere from ten to one hundred times less effectively.

To maintain optimal accuracy, OCXOs may require addressing factors such as crystal aging or undergo periodic calibration depending on application needs, typically every six months to a year.

More Physical Abilities of an OCXO

OCXOs are generally bulkier compared to standard crystal oscillators due to their integrated components:

• Incorporation of the crystal oscillator
• Presence of a heater
• Control circuitry
• Thermal insulation surrounding the oscillator

The heater operates independently from the oscillator and can demand substantial power at startup, often consuming around an amp or more before decreasing as the system reaches optimal temperature.

Temperature-Compensated Crystal Oscillators (TCXOs)

TCXOs present a different approach by addressing the primary challenges of quartz crystals. Their performance can significantly enhance temperature stability, commonly achieving enhancements of 10 to 40 times that of traditional crystal oscillators. Although maintaining stabilities better than ±1.5 ppm in a 0 to 70°C range is challenging and comes with increased costs.

Power Dissipation: With added circuitry, TCXOs tend to consume more power than standard versions. Additionally, they experience a short startup time before stabilizing, potentially taking about 100 ms or longer based on the design.

TCXO Package: TCXOs are available in various package designs correlating to user needs, with the most common construction featuring a small printed circuit board (PCB) in a metal casing suitable for circuit board installation. Since the crystals are sealed, external packaging sealing is often non-critical.

Sizes such as 5 × 3.2 × 1.5 mm or 5 × 3.5 × 1 mm are prevalent, with smaller options accessible if necessary.

Output Format and Level: Many TCXOs are used to interface with digital circuits and commonly generate what is known as a clipped sine wave, which is effective for logic circuits. However, using a logic buffer may be prudent to ensure signal quality; sine wave outputs must be chosen initially, as options may be limited.

Power Requirements: Power consumption varies by device. Many TCXOs run on 3V supplies while drawing as little as 2 mA, though this fluctuates based on specific device characteristics and manufacturers.

4 Common Types of TCXOs

While most refer to these devices as TCXOs, more detailed classifications exist depending on their temperature compensation methods.

ADTCXO: This analog-digital TCXO type is common in cell phones, using analog technology for temperature corrections without abrupt phase shifts.

DTCXO: Utilizing temperature sensors, this digital TCXO processes logic with digital circuits and lookup tables to derive correction factors, converting results to analog signals via DAC.

DCXO: Integrated directly with host processors, this type minimizes costs by incorporating corrections into the overall equipment's processing tasks.

MCXO: This microprocessor-driven TCXO offers advanced processing capabilities for precise compensation across varying conditions, which comes with a higher price point.

TCXOs are favored for applications demanding accurate frequency sources; they are typically more compact and affordable than OCXOs, making them ideal for portable devices that require reliable frequency output.