PROPERTIES OF INTERDIGITAL TRANSDUCERS IN RELATION TO THE SUBSTRATE CRYSTAL SYMMETRY

Exclusively from the crystal symmetry, we obtain results concerning pr operties of interdigital transducers radiating Rayleigh waves along hi gh-symmetrical crystal directions. Three cases of high-symmetrical dir ections are considered: the propagation direction is parallel to a rot ation axis, perpendicular to a mirror plane, or parallel to a rotoinve rsion axis of the substrate crystal. In respect to the symmetry proper ties of the piezoelectric tensor related to a mirror operation on a pl ane perpendicular to the propagation direction, the three original cas es can be concentrated in three symmetry cases. The crystal symmetry d etermines the properties of the piezoelectric tensor, and consequently the wave excitation by transducers. These considerations enable one t o ascertain the positions of the transduction centers and the influenc e of internal reflections to the amplitude characteristics. Furthermor e, we are able to discuss natural single-phase unidirectional transduc er (NSPUDT) effects in layer structures, the old question ''crossed-fi eld or in-line model,'' and the existence of cross-coupling of waves, similar to multistrip couplers, in interdigital transducers (IDT's). T he theoretical predictions are compared with our own and other authors ' experimental results.