Stabilization Time and Steady State

      This defines as the time taken to reach a certain level of stability after a long period of being turned off.  Oven power reaches the specified maximum, after which it cuts back to reach steady state when the oven has reached its operating temperature.  Power consumption for OCXOs is typically around 5W at warm-up and 1.5W at steady state, depending on size.

Retrace

      Retrace is the frequency error after power is applied, comparing to the previous value and aging rate before power was removed.  When measuring the retrace, the normal period that the OCXO is powered off is 24 hours, and the normal period powered up is to have sufficient time to allow complete thermal equilibrium.  Good retrace is obtained by proper design of the oscillator, oven mechanics, and crystal resonator.  This is of the order of ± 20 to ±50 ppb. There is significant variation in these characteristics from crystal to crystal.  In addition to the crystal related effected described above, thermal stresses from heating and cooling the oven structure can also contribute to the retrace, and changes in aging rate.  In most applications, OCXOs are continuously powered up.  This being the case, aging is the critical characteristic with turn-off/turn-on characteristic being of little or no significance.  However, when applications require frequent turn-off, an additional series of characteristics (such as Retrace) should be considered.

Double Rotated (SC-cut) Crystals

       While most high stability crystal oscillators use AT-cut crystals, SC-cut crystals are often used in the highest stability OCXO models.  An SC-cut crystal is one of a family of double rotated crystals (quartz crystals cut on an angle relative to two of the three crystallographic axes).  Others in the family include the IT-cut and FC-cut.  The SC-cut represents the optimum double rotated design as its particular angle provides maximum stress compensation.   Following is a comparison between double rotated and AT-cut crystals.

            Advantages of the SC-cut crystals:

Improved aging – For a given frequency and overtone (e.g. 10 MHz, 3^rd overtone), the SC-cut crystal provides 2 to 3 times aging improvement relative to AT-cut.

Thermal transient compensated - Allowing faster warm-up in OCXOs

Phase noise – For a given oscillator design for a particular crystal frequency and overtone, the SC-cut crystal provides higher Q and associated improved phase noise characteristics.

Planar stress compensated - Smaller changes in frequency due to edge forces and bending

Static and dynamic F vs. T - Allowing higher stability OCXO and MCXO

Better F vs. T repeatability - Allowing higher stability OCXO and MCXO

Far fewer activity dips

Lower drive level sensitivity

Lower sensitivity to radiation

            Disadvantages of the SC-cut crystals:

Cost - Because of difficulties associated with tightly-controlled angle rotations around two axes in the manufacture of SC crystals vs one axis for the AT, the SC crystal is significantly higher in cost than that of an AT of the same frequency and overtone.

Pullability - The motional capacitance of an SC crystal is several times less than that of an AT of the same frequency and overtone, thus reducing the ability to "pull" the crystal frequency.  This restricts the SC crystal from being used in conventional TCXOs and VCXOs, or even in oven controlled oscillators requiring the ability to deviate the frequency of oscillation by any significant degree.

      In summary, the suitability of double rotated crystals for use in crystal oscillators is essentially restricted to those oven controlled applications where the improved aging, warm-up, and close-in phase noise characteristics justify a significant cost increase.

Allan Variance

      Allan Variance, also known as short-term stability, is similar to phase noise except that it is based in the time domain instead of the frequency domain.  Typical numbers for a 10 MHz OCXOs are shown in the following Table.