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An important effect in TCXOs is the
interaction between the frequency adjustment during calibration and the f vs.
T stability. This phenomenon is called
the trim effect. In TCXOs,
temperature-dependent reactance variations are used to compensate for the
crystal's f vs. T variations. During
calibration, the crystal's load reactance is varied to compensate for the
TCXO's aging. Since the frequency
versus reactance relationship is nonlinear (see next page), the capacitance
change during calibration moves the operating point on the frequency versus
reactance curve to a point where the slope of the curve is different, which
changes the compensation (i.e., compensating for aging changes the f vs. T
stability). The next page shows how,
for the same compensating CL vs. T, the compensating f vs. T
changes when the operating point is moved to a different CL. Shown above are test results for a “0.5 ppm
TCXO” that had a ±6 ppm frequency-adjustment range (to allow for aging
compensation for the life of the device).
When delivered, this TCXO met its 0.5 ppm f vs. T specification;
however, when the frequency was adjusted ±6 ppm during testing, the f vs. T
performance degraded significantly.
The 0.5 ppm TCXO was shown to be a 2 ppm TCXO.
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In specifying a TCXO, it is important to
require that the f vs. T stability include the hysteresis and trim effects.
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R. L. Filler, V. J.
Rosati, S. S. Schodowski, and J. R. Vig, "Specification and Measurement
of the Frequency Versus Temperature Characteristics of Crystal
Oscillators," Proc. 43rd Annual Symposium on Frequency Control, pp.
253-255, 1989, IEEE Catalog No. 89CH2690-6.
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