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Shown above are frequency vs. temperature
characteristics of the b-mode and c-mode of a 10 MHz 3rd overtone SC-cut
resonator. The b-mode’s f vs. T is
monotonic, with a slope of about -25.5 ppm per degree C. The c-mode is temperature compensated; its
exact f vs. T characteristic depends on the angles of cut, overtone, contour,
etc. The b-mode’s frequency is about
9.4% higher than the c-mode’s (but it can range from ~9% to 10%, depending on
design).
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Both modes can be excited simultaneously, without
the modes interfering with each other.
The b-mode frequency can be used to instantaneously indicate the
temperature of the resonator’s active volume. Dual mode excitation of the b-mode and
c-mode frequencies has been used for temperature compensation. In later work, however, dual mode
excitation of the fundamental and third overtone c-modes was shown to be
superior - see “Resonator Self-temperature Sensing” and subsequent pages in
Chapter 2.
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J. A. Kusters, M. C.
Fischer, J. G. Leach, "Dual mode operation of temperature and stress
compensated crystals," Proc. of the 32nd Annual Frequency Control
Symposium, 1978. pp. 389-397.
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