Cavity Resonator for Piezoelectric Surface Excitation

Abstract

The three-dimensional problem of surface excitation of acoustic waves in a piezoelectric crystal is examined. For practical conditions, it is found that the excitation of a given acoustic mode is proportional to the scalar product of the exciting electric field and the “excitation vector,” which is determined by the material constants of the crystal. The direction of this excitation vector is evaluated for some common orientations in crystal quartz. Details are given of a new type of double-tuned cavity resonator, which, by suitable choice of one or the other of its two resonant modes, is capable of providing an electric field at the surface of the piezoelectric crystal that has a substantial component along any preferred excitation direction. A field-probe technique has been used to determine both the amplitude distribution of the exciting electric field in the cavity and the resulting acoustic wave in the crystal. It is found that constant phase of excitation over the cross section of the acoustic beam is a prerequisite for obtaining good exponential-decay patterns for successive reflection of many echoes in the crystal. Selection of the appropriate resonant mode of the cavity has enabled excellent results to be obtained for the decay patterns, using various orientations of crystal quartz.