Numerical investigation of PSAW propagation in waveguides

A full wave PSAW waveguide theory is used to calculate PSAW waveguide propagation properties on orientations of quartz, lithium tantalate, and lithium niobate. Only structures where the wave velocities nominally inhibit sideways radiation from the guide, proper waveguides, are investigated. Curves showing losses versus waveguide width are given for several surface conditions outside the waveguide. Even for proper waveguides the losses due to bulk wave excitation at the edges of the guides may be substantial, depending on the mechanical and electrical loading of the two adjoining surfaces. Especially in quartz this effect is pronounced, and total loss differences of up to a factor of ten are found for waveguides only differing in the mechanical loading outside the guide. The losses are in general found to be too high for use of PSAW in waveguide coupled filters. For STW on quartz it is however not far from being possible. Losses due to coupling to PSAW or the Rayleigh wave propagating at a large angle to the guide, are not found to be a problem.