Highly Stable, Ovenized Bulk Shear Mode Resonators

In previous work we have demonstrated a parabolic frequency dependence on temperature for specific cuts of lithium tantalate excited by shear mode ZnO transd~cers(l*~). These cuts confirmed the validity of the computer model used to determine the first order zero temperature coefficient propagation directions(3s4). This work also demonstrated that nonpiezoelectric microwave resonators exhibiting temperature stability of an overtone frequency could be produced. Our current work is an extension of this effort with three goals: (1) Find more useful cuts in terms of providing a higher temperature turnover appropriate for stabilization by means of a themstated oven. (2) Provide increased vibration resistance by identifying crystal cuts which have minimum internal piezoelectric coupling with the requisite temperature behavior as in (1). (3) Permit the highest loaded Q with the requirements provided for in (1) and ( 2 ) . Toward these goals we have examined the temperature frequency characteristic of a new singly rotated cut of lithium tantalate. Several identical cuts were tested at a number of different microwave frequencies. The turnover region €or this new cut is from +6OoC to +80°C. The resonator plate orientation for which these results were obtained is a single rotation of the plate about x away from z to an angle near loo. Loaded Q values in excess of lo4 at 2 GHz have been measured. This value is believed to be limited by material defects distributed throughout the bulk of the crystal. These results show temperature stable high overtone bulk acoustic resonators (HEARS) with an application potential substantially greater than those shown before with a frequency stability exceeding parts per million with relatively simple temperature stabilization.