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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. When an OCXO is turned on, there can be a
significant thermal-transient effect.
The above figure 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.
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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.
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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 (see “Apparent Hysteresis”
later in this chapter). 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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A. Ballato, and J.
R. Vig, "Static and Dynamic Frequency-Temperature Behavior of Singly and
Doubly Rotated, Oven-Controlled Quartz Resonators," Proc. 32nd Annual
Symposium on Frequency Control, pp. 180-188, 1978, AD-A955718.
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