Understanding the Differences Between TCXO and OCXO

When designing circuit boards, two types of crystal oscillators are commonly used - constant temperature (OCXO) and temperature-compensated (TCXO) oscillators. 

Knowing when to use each type can help you optimize your circuits for better performance. In this article, we will explore the differences between 

TCXO and OCXO from three perspectives.

Definition

OCXO stands for Oven Controlled Crystal Oscillator. As the name suggests, OCXOs use a constant temperature bath to maintain a stable environment for the quartz crystal resonator, minimizing frequency changes caused by temperature fluctuations. Typically, a differential series amplifier that uses a thermistor "bridge" is used to achieve temperature control.

TCXO stands for Temperature Compensated X'tal (crystal) Oscillator. It is a crystal oscillator that uses electronic means to compensate for frequency-temperature offsets in the crystal component, resulting in a more stable device. The TCXO compensates for the frequency-temperature stability of the crystal element by using the change of the crystal load reactance with temperature.

Working Principles

OCXOs keep the temperature of the oscillator constant to minimize the output frequency change caused by temperature changes. The crystal oscillator's oscillation frequency changes with temperature, so keeping it at a constant temperature improves its phase-frequency characteristics.

TCXOs reduce the variation of oscillation frequency caused by ambient temperature changes through their additional temperature compensation circuit. The oscillator compensates the frequency drift of the resonator due to changes in ambient temperature by changing the load capacitance in the oscillating circuit, which changes with temperature.

Measurement Accuracy

OCXOs typically have a frequency stability more than two orders of magnitude higher than that of TCXOs. For example, the frequency stability of a TCXO is generally around -7, while an OCXO can reach -9. This makes OCXOs ideal for high-end measuring instruments, such as frequency counters, signal generators, network analyzers.

TCXOs have better start-up characteristics. Even with the best heating element, OCXOs still require a heating process that can take up to five minutes to reach -7 and even up to a day to reach -9. Therefore, they are not suitable for devices that need to work as soon as they are turned on.

Conclusion

In summary, both TCXO and OCXO are active crystal oscillators that require power to work. OCXOs use a constant temperature bath to keep the temperature of the oscillator constant, while TCXOs use electronic means to compensate for frequency-temperature offsets in the crystal component. While OCXOs have better frequency stability, TCXOs have better start-up characteristics. Knowing when to use each type of oscillator will help you design circuits that perform optimally.

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