OCXO Datasheet, PDF (7/7 Pages) MICORO CRYSTAL SWITZERLAND – Oven Controlled Crystal Oscillator

Application Note
the 70°C to 90°C temperature range. Based upon (a) the desired 10°C difference between the highest
operating ambient temperature and the crystal turnover temperature, and (b) the manufacturing toler-
ance of crystal turnover temperatures, these crystals are best suited for maximum operating ambient
temperatures of 50°C to 75°C. However, the upper temperature turnover point of the IT crystal (“C” in
Figure 7) is well suited to higher temperature operation and thus the IT crystal is a logical choice for high
stability oven controlled oscillators having a maximum operating temperature in the 85°C to 95°C range.
Note that while SC and IT crystal curves are relatively flat at elevated temperatures, their frequency falls
off rapidly at low temperatures. Thus, while they serve well in high stability HIF oven controlled oscilla-
tors, they are generally not well suited for other types of stable crystal oscillators.
5. Orientation Sensitivity (tip-over). When the physical orientation of an oscillator is changed, there is a
small frequency change (typically not more than several parts in 10^-9 for any 90 degree rotation), due to
the change in stress on the crystal blank resulting from the gravitational affect upon the crystal supports.
Tip-over is expressed in 10^-9/g where one g represents one half of a 180° orientation change. The SC
crystal is less frequency sensitive to orientation change than is the AT. However, the tip-over difference
between AT and SC crystals is not consequential for most applications and this characteristic is usually
not a specification consideration.
6. Spurious Under Vibration. When a crystal oscillator is subjected to vibration, spurious frequencies are
generated, offset from the frequency oscillation by the frequency of vibration. The amplitude of these
spurious outputs is related to the amplitude of vibration, the mechanical design of the crystal support,
and the mechanical design of the oscillator. The SC crystal produces lower amplitude spurious ouput
under vibration than does the AT; however, this characteristic is determined more by the mechanical
designs of the crystal and oscillator than by crystal cut.
Disadvantages of SC Crystals:
1. 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.
2. 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 re-
quiring 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 character-
istics justify a significant cost increase.
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