Distortion of transverse mode symmetry in SAW transversely coupled resonators due to natural SPUDT effects

A anomaly was observed in experiments associated with including higher-order modes in a new model for SAW transversely coupled resonator filters. Laser probing revealed that in addition to exciting modes of the expected symmetry, (a) a transversely symmetric electrical drive excited anti-symmetric transverse modes and (b) a transversely anti-symmetric electrical drive excited symmetric transverse modes. This effect is totally unexpected based on the commonly used scalar waveguide models and on the symmetry of the slowness surface of the ST family of quartz cuts. The observed effect clearly represents a failure of a strictly scalar theory. In this paper it will be shown that the same asymmetry that is associate with the natural SPUDT effect skews the electrical coupling such that all modes have both symmetric and antisymmetric components of electrical coupling. This paper predicts the existence of a similar distorting effects and corresponding errors in the commonly used scalar diffraction theory for SAWs