Thermal Hysteresis

       It is a measure of a TCXO to repeat the frequency versus temperature data over multiple temperature cycles.  Here the frequency of a TCXO is measured at one temperature.  The temperature is changed and then returned to the original temperature and the frequency is measured again.  The two frequencies are not the same.  The difference between the two frequencies is called “thermally induced hysteresis”.  This phenomenon is present even if the unit is allowed to stabilize at the same temperature for a long time.  The value of this thermal induced hysteresis is normally of the order of ±0.1 ppm for a good TCXO.

Thermal Transient

        Thermal transient occurs when the rate of temperature change is high enough for the frequency to no longer tack the well-behaved curve that is generated when measured with slow temperature changes.  An acceptable rate of the temperature change would be of the order of 0.5 C per minute.  This effect is in a large part due to the transient response of the crystal resonator, and the separation between resonator and temperature sensing devices within the oscillator.  In an OCXO, it can also depend on the stability and gain of the error amplifier used in the temperature controller.  Typical values are less than ±0.2 ppm.

     The testing and compensation accuracies of TCXOs can be adversely affected by the thermal-transient effect.  As the temperature is changed, the thermal-transient effect distorts the static F vs. T characteristic, which leads to so-called apparent hysteresis.  The faster the temperature is changed, the larger is the contribution of the thermal-transient effect to theF vs. T performance. F vs. T performance.

Aging

     In clock oscillators with moderate temperature stability, aging is usually of little consequence. However, in highly temperature stable TCXOs, crystal aging becomes a significant factor in the oscillator's overall frequency error.  Therefore, it is very common for TCXOs to employ specially processed crystals in evacuated glass or cold weld holders.

Shock

      Shock is defined as a sudden powerful blow.  A typical shock number for TCXOs is 100g.

Mechanical Trim and EFC (electrical frequency control)

      Mechanical trim allows the frequency to be adjusted via an internal potentiometer (pot).  The pot is accessed through a sealed or unsealed hole.

     EFC (electrical frequency control) requires an external circuit to adjust the frequency.  The external circuit usually consists of a pot or DAC.  The power for this circuit can be applied via an external voltage source supplied by the customer or an internal reference voltage supplied by the manufacturer.

5. Oven Controlled Crystal Oscillators

     In an OCXO the crystal and other temperature sensitive circuitry is placed in a temperature controlled structure.  The idea is to keep the crystal at a stable temperature higher than the highest ambient temperature to which the OCXO will be exposed.  For best results, the oven is set to the resonators turnover temperature.  Either AT-cut or SC- cut crystal resonators may be used.  The SC-cut crystal resonator offers the best overall performance, while the AT-cuts offers lower cost.

    The  primary reason behind controlling the temperature is to remove the effect of temperature induced anomalies.  All quartz crystal resonators are associated with those thermal anomalies that only allow compensation (or predictability) to within ±0.1 ppm.  The other reason is to allow the use of higher overtone crystals which are not very pullable but very stable, to be set at particular frequency by controlling the temperature.  The use of higher overtone crystals also results in improved short-term stability resulting from higher Q of the resonators, and improved long-term performance resulting from the increased quartz mass of the resonators.