Introduction to Quartz Frequency Standards - Dynamic Frequency versus Temperature Stability

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Changing the temperature surrounding a crystal unit produces thermal gradients when, for example, heat flows to or from the active area of the resonator plate through the mounting clips. The static f vs. T characteristic is modified by the thermal-transient effect resulting from the thermal-gradient-induced stresses [22]. When an OCXO is turned on, there can be a significant thermal-transient effect. Figure 22 shows what happens to the frequency output of two OCXOs, each containing an oven that reaches the equilibrium temperature in six minutes. One oven contains an AT-cut, the other, an SC-cut crystal. Thermal gradients in the AT-cut produce a large frequency undershoot that anneals out several minutes after the oven reaches equilibrium. The SC-cut crystal, being "stress-compensated" and thereby insensitive to such thermal-transient-induced stresses, reaches the equilibrium frequency as soon as the oven stabilizes.

Figure 22. Warm-up characteristics of AT-cut and SC-cut crystal oscillators (OCXOs).

In addition to extending the warmup time of OCXOs, when crystals other than SC-cuts are used, the thermal-transient effect makes it much more difficult to adjust the temperature of OCXO ovens to the desired turnover points, and the OCXO frequencies are much more sensitive to oven-temperature fluctuations [22].

The testing and compensation accuracies of TCXOs are also 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 apparent hysteresis [26]. The faster the temperature is changed, the larger is the contribution of the thermal-transient effect to the f vs. T performance.

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