Oven controlled quartz crystal oscillators - commonly referred to as OCXOs - achieve reference frequency stability by housing the quartz crystal resonator in its own stable oven.
By maintaining a stable internal temperature while operating over the application’s ambient temperature range, an OCXO provides a very stable frequency source.
Unlike temperature compensated crystal oscillators - “TCXOs” - an OCXO’s oven temperature is set to a temperature above the highest operational ambient temperature to allow maintenance of temperature control, often via the use of a thermistor temperature sensor. Having established an appropriate oven temperature, the crystal will be specified to have a turning point at which the frequency vs. temperature slope is near zero at the temperature the sensor sees in the oven.
An OCXO oven usually includes one or more heating elements and, because other electrical components in the circuit are also susceptible to drift, the entire oscillator circuit is housed inside the oven. A thermistor temperature sensor in a closed-loop circuit is the most commonly used method to control power to the heater and precisely control oven temperature. Because the oven operates above the full operating temperature range, OCXOs require a warm-up period after power is applied. During warm-up, more power is required as the unit heats from room ambient to the higher internal oven temperature. Frequency will drift during warm-up until the OCXO reaches set operating temperature. OCXO oven temperature is typically set to 75°C to 80°C for commercial/industrial applications, but higher and/or wider ranges (e.g., 90°C to >100°C) can be specified.
OCXOs may utilize either common AT-cut or SC-cut (stress compensated) quartz crystals. First developed in 1974, SC-cut crystal OCXOs are better suited to elevated operating temperatures and provide low phase noise and better aging characteristics.
The short term frequency stability of an OCXO is typically 1 x 10-11 over a few seconds and roughly 5 x 10-10 per day. Long term OCXO aging is generally assessed by 100% testing over a period of days.
In summary, OCXOs offer the benefits of low phase noise, tight frequency vs. temperature stability and excellent long term aging performance, particularly when SC-cut crystals are utilized.