OCXO Datasheet, PDF (6/7 Pages) MICORO CRYSTAL SWITZERLAND – Oven Controlled Crystal Oscillator

Application Note
Double Rotated (SC and IT Cut) Crystals
While most high stability crystal oscillators use AT Cut Crystals, SC and IT Cut Crystals are often used in the
highest stability 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, but
similar performance is achieved with the IT Cut.
Following is a comparison between double rotated (referred to simply as SC for convenience) and AT Cut
crystals.
Advantage of SC Crystals:
1. Improved Aging. For a given frequency and overtone (e.g. 10 MHz, third overtone), the SC crystal pro-
vides 2:1 to 3:1 aging improvement relative to AT crystals.
2. Warm-up. In oven controlled oscillators with a given oven design and turn-on power, the SC crystal
achieves its “final frequency” in considerably less time than does the AT crystal.
3. Phase Noise. For a given oscillator design, crystal frequency and overtone, the SC crystal provides
higher Q and associated improved phase noise characteristics. This improvement applies primarily close
to the carrier as the noise floor is determined by circuit design rather than the crystal.
4. High Operating Ambient Temperature. Figure 7 shows the relative frequency-temperature characteris-
tics of AT, IT and SC crystals. The upper temperature turnover point of the AT crystal (“A” in Figure 7) and
lower temperature turnover point of the SC crystal (“B” in Figure 7) are optimally in
AT
A
B
C
IT
SC
Figure 7.
-50
-25
0
25
50
75
100
125
150
Temperature
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