OCXO Datasheet, PDF (3/7 Pages) MICORO CRYSTAL SWITZERLAND – Oven Controlled Crystal Oscillator

Application Note
Warmup with AT cut crystals
When an oscillator is initially turned on at room temperature the frequency is extremely high relative to the
output frequency after the oven stabilizes, typically by 30x10^-6. This is simply due to the fact that the frequen-
cy of an AT cut crystal is considerably higher at room temperature than at its upper turnover temperature. As
the oven warms up, the crystal frequency rapidly decreases. In standard Vectron oscillators, the oven balanc-
es in 10-15 minutes but the crystal displays a rubberband effect and overshoots its final frequency per Figure
3, prior to stabilizing. Typically, relatively high degree of stability is achieved within 30 minutes after turn-on;
this time can be reduced to less than 5 minutes in special fast warm-up designs.
Figure 3.
3
2
1
0
-1
Turnover Temperature
10
20
30
Time (Minutes)
The oven operating temperature (crystal turnover temperature) must be several degrees higher than the high-
est ambient temperature in which the oscillator is to operate in order that the oven may maintain good control
(considering the internal heat rise generated by the oscillator itself ).
However, there are disadvantages associated with high oven temperature operation. First, the crystal’s frequen-
cy vs. temperature characteristic is sharper with higher turnover crystals resulting in more sensitivity to minute
changes in oven temperature as shown in Figure 4.
Figure 4.
Crystal with
60°C
Turnover
Crystal with
80°C
Turnover
Crystal with
100°C
Turnover
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