The frequency stability of quartz oscillators is affected by temperature fluctuations. To ensure high frequency stability, there are temperature-controlled and -compensated designs that ensure high temperature consistency. High- constant, high- performance crystal oscillators therefore use temperature-controlled circuits that keep the crystal and critical circuits at a constant temperature.
In the thermostated crystal oscillator, Oven Controlled Crystal Oscillator (OCXO), the crystal and oscillator circuits are in a closed, heated enclosure (oven) where the temperature is kept highly constant. The temperature is about 10 to 20 degrees above the highest ambient temperature.
The quartz selected for these applications has a temperature dependence that reverses near the oven temperature. The consistency of the quartz is very high at this temperature and changes only slightly with rising and falling temperature. This has the advantage that temperature variations of the furnace have only a very minor effect.
Oven-controlled crystal oscillators have a warm-up time of several minutes before they reach their high frequency constancy. By combining the highly stable oven temperature and the inverse behavior of the crystals, such OCXO circuits achieve frequency stabilities of 0.0001 ppm, corresponding to `10^-10`.
To increase frequency stability, OCXO oscillators can be readjusted with the signal from atomic frequency standards. The more stable atomic clock signal replaces the resonant frequency of the OXCO oscillator, giving the OXCO frequency a much higher frequency stability.