I. Introduction
The design of Surface Acoustic Wave (SAW) devices requires flexible and reliable software tools offering fast and accurate computations. The majority of the tools involve simulations of a semi-infinite unit cell with periodic boundary conditions, the derivation of the Harmonic Admittance (HA), and the extraction of the Coupling-of-Modes (COM) parameters for the later simulation of SAW devices. The mathematical models used there are commonly based either upon a finite and boundary element methods combination (FEM/BEM) [1], [2] or upon a full-FEM approach [3]–[5]. The second seems more suitable in case of multi-layered structures which are used in combination with heavy electrodes for SAW filters and high temperature SAW sensors. Indeed, the full-FEM model remains easy to use, whereas FEM/BEM becomes increasingly complicated due to the initial computation of sophisticated multi-layered Green's functions. The full-FEM approach, on the other hand, requires a truncation of the semi-infinite computational domain. The truncation in its turn must be performed in a way that allows spurious non-physical reflections at an artificial lower boundary to be avoided.