Ahh the mysterious OCXO. What the heck is an OCXO anyway? What's it good for? When should you use one in your electronic system design? These questions and more will all be answered in this post.
No, OCXO doesn't stand for Octopus' Conduct Xylophone Orchestras if that was on your mind. OCXO actually stands for Oven Controlled Crystal (Xtal) Oscillator. OCXO are the top performing type of quartz crystal oscillator at the expense of power. Other types of quartz crystal oscillators include TCXO, VCXO, and just plain XOs. You can read more about all the different types of quartz crystal oscillators here.
How an OCXO Oscillator Works (and Benefits)
A finely tuned quartz crystal (to a specific center frequency) is placed in an oven-like device. This oven is heated and controlled with high-precision to a temperature that matches the upper turning point frequency vs. temperature.
The whole reason for the oven (and maintaining the temperature at the turn over temperature) is to protect the quartz crystal from external temperature changes. These external temperature changes can cause the quartz crystal to drift off its desired center frequency.
The way the quartz crystal is cut inside the OCXO can also impact its performance. Quartz crystals are either cut to an "AT cut" or "SC cut". The main advantage of the SC cut is that it provides overall better frequency stability. This is because an SC cut has a lack of transient response for dynamic temperature changes which is due to the lack of stress sensitivity.
An OCXO can see around 100 times improvement in frequency vs temperature stability compared to an un-ovenized quartz crystal. OCXOs are known for providing superior short-term stability.
How Stable Are OCXO Oscillators?
The table below lists the stability hierarchy of common oscillators/atomic clocks from most stable to least stable.
| Stability | Type | Relative Accuracy |
| Highest | Cesium Atomic Clock | 1 x 10-13 |
| High | Rubidium Atomic Clock | 1 x 10-11 |
| Middle | OCXO |
1 x 10-9 1 x 10-7 |
| Low | TCXO | 1 x 10-6 |
| Lowest | XO | 1 x 10-4 |
As you can see, an OCXO is right in the middle when it comes to stability of common oscillators and clocks.
However, it's common for atomic clocks to contain an OCXO. This allows for atomic clocks to offer good long-term and short-term stability. An atomic clock alone typically only offers longer term stability. Pairing an OCXO with an atomic clock makes for a great frequency reference solution.
The Biggest Disadvantage of OCXO Oscillators
As we mentioned before, higher power consumption is one of the major disadvantages of OCXOs. This disadvantage typically makes OCXOs not a very suitable option for battery applications.
Additionally, OCXO require a significant warm-up time of a few minutes before it reaches its specified temperature range after powering on. This is unlike XOs and TCXOs that can instantly reach their specified frequency stability on start-up.
Power consumption for OCXOs is of course higher during start-up/warm-up periods, but slowly settles as it reaches it's steady state of operation. Warm-up and Steady-state power consumption are typically listed on OCXO datasheets. It's also worth noting that OCXO power consumption is inversely related to the external temperature.
Speaking of Temperature...
What happens if the external temperature of the OCXO exceeds the maximum operational temperature? Well, the regulation of the oven becomes ineffective. The internal power dissipation of an OCXO requires the quartz crystal to be used at a slightly higher temperature than the maximum operational temperature. Therefore, the OCXO will rely on the crystal itself for temperature stability if this occurs.
Synchronizing OCXO and Clocks
Many times OCXOs are synchronized with a clock that has a higher degree of accuracy. This connection usually has 2 states of operation...
- Synchronized state
- Hold over state
When the clock and the OCXO become unlocked in a system, the system switches over to a "hold over" state. In this situation, the system needs to depend on the OCXOs internal performance. The hold over stability during this time depends on many factors, but the main contributors are long-term stability (aging) and temperature dependence.
A good example of this is when a satellite receiver (such as GPS, GLONASS, GALILEO, or BEIDOU) is disciplining an OCXO by outputting 1 pulse per second (PPS). This PPS is then steering the OCXO to synchronization. If the satellite system remains locked, all clocks in the network can achieve great long-term stability (typically 1 x 10-12). But if the system becomes unlocked for any period of time, the OCXO will help maintain connection/communications during the "outage".
Discover the latest in GPS Disciplined OCXO technology. Download the full Atlas 1PPS datasheet.
