Frequency-temperature characteristics of quartz crystal units of different cuts operating over a wide temperature range including helium temperatures

Investigation of frequency vs temperature dependence of quartz crystal elements of singly- and doubly-rotated cuts during excitation of B- and C-type vibrations in them has been carried out over the temperature range from 4 to 400 K. The plate orientation accuracy was ±30". Isonormal quasitransversal vibrations and pure shear vibrations have been exited in metallized piezo-electric elements at frequency 5 MHz and 10 MHz. When setting up equations it is necessary to take into account the change of curvature sign due to relaxation and to make operations with generalized frequency-temperature characteristics (FTCs). FTCs of yxbl/+15°/-35°19\´ crystal units (type B-vibrations) have extrema at T=86 K and 229 K. FTCs of crystal units of the same cut in which vibrations of type C have been excited, have no extrema except for the maximum at T-0 K. Crystal units of other cuts have such FTC features as maxima, minima and flat portions with the inflection