Will a crystal oscillator operate outside its specified temperature range? This is a common question and the short answer is: yes, but it's complicated.
If, for example, a crystal oscillator is specified over 0 to 60 °C, it will most likely perform without any issues over -40 to 85 °C. But it's possible that it will fall outside its specified stability. This may not matter if the application needs just a stable frequency rather than an accurate frequency.
Below are test results of the same oscillator over two different temperature ranges. The one on the left is from -20 to 70 °C. As you can see, it is very stable. The one on the right is from -40 to 85 °C and is not as stable at the extreme temperatures.
Simply put, if temperature stability is vital in your system, then operating within specifications is necessary.
Problems Caused by Operating Outside the Specified Temperature Range
We wouldn't recommended operating a crystal oscillator outside of its specified temperature range for a variety of reasons. The most notable reason is the greater frequency drift that is possible due to the increased temperature extremes.
Another phenomena that can occur is accelerated crystal aging, which can impact both stability and frequency accuracy.
Some of the most serious problems that can occur are activity dips. In an OCXO, this could simply be due to running the oscillator above the oven’s setting temperature. In an activity dip, the oven will shut off, as shown below.
Activity dips can also cause the crystal oscillator to stop oscillating completely at a specific temperature. Manufacturers usually design around this problem, but can’t guarantee it beyond the specified range.
For system designers who need to be careful about power usage, it's important to know that an OCXO will also usually draw more current to keep the oven going at colder temperatures. The power consumption of an oscillator may be acceptable at -20 °C but not at -40 °C.
In Conclusion: Try to Keep within Oscillator Operating Temperature Range
To sum it all up, while you can operate a crystal oscillator outside of the specified temperature range, we don't recommend it.
The best course of action is to know the needed temperature range ahead of time and stick with it, if possible. If you run a unit outside of manufacturer’s specifications, a variety of issues can arise, from the merely annoying to the system crippling.
Want to learn even more about what makes these devices tick? Check out this guide to understanding the types of quartz crystals inside OCXOs.