New designs for resonators and filters using lithium tantalate

The results of investigations made into improving the characteristics of fundamental and overtone mode resonators are presented. Using the stroboscopic topography technique, it is shown that the rather poor Q factors of the fundamental thickness shear resonators are due to non-stationary plate modes coupled to the fast shear. The use of bar shaped thickness shear resonators permits higher Q factors to be obtained. To obtain overtone resonators with a zero first order temperature coefficient, (TC) doubly rotated cuts, the compensation of the TC of the overtones of the X cut by a layer of silica, and the use of the lateral field excitation are considered. This latter type of resonator is very well adapted to lithium tantalate and permits, using the plano-convex geometry, very large Q factors for overtones with very elevated ranks to be obtained