Theoretical analysis of relations between directivity of SAW transducer and its dispersion curves

Coupling-of-modes (COM) analysis is applied to the analysis and design of low-loss SAW (surface acoustic wave) filters incorporating SPUDTs (single-phase unidirectional transducers). Coefficients of the COM equation-excluding electrostatic capacitance, which is evaluated by the finite element method (FEM) as an electrostatic problem-are evaluated from four frequencies at the stop band of the dispersion curve of the SAW transducer in open- and short-circuited states, and the FEM was used to evaluate these frequencies. Closed-form expressions for directivity in terms of these coefficients are obtained. Saturation phenomena of directivity under the condition of φ-φ≠π/4 and the relation between the directivity and the frequencies at the sides of the stop band are described. Experimental results are given for the thickness-difference-type SPUDTs and SAW filters incorporating SPUDTs on 128° Y-X LiNbO₃. Comparison with experiments shows that the theoretical predictions are quite good