Properties of interdigital transducers in relation to the substrate crystal symmetry

Exclusively from the crystal symmetry, we obtain results concerning properties of interdigital transducers radiating Rayleigh waves along high-symmetrical crystal directions. Three cases of high-symmetrical directions are considered: the propagation direction is parallel to a rotation axis, perpendicular to a mirror plane, or parallel to a rotoinversion axis of the substrate crystal. In respect to the symmetry properties of the piezoelectric tensor related to a mirror operation on a plane perpendicular to the propagation direction, the three original cases can be concentrated in three symmetry cases. The crystal symmetry determines the properties of the piezoelectric tensor, and consequently the wave excitation by transducers. These considerations enable one to ascertain the positions of the transduction centers and the influence of internal reflections to the amplitude characteristics. Furthermore, we are able to discuss natural single-phase unidirectional transducer (NSPUDT) effects in layer structures, the old question "crossed-field or in-line model", and the existence of cross-coupling of waves, similar to multistrip couplers, in interdigital transducers (IDTs). The theoretical predictions are compared with our own and other authors' experimental results.