An equivalent circuit for simulating Love mode acoustic wave transducers: comparison of simulation and experimental results

Abstract

A simulation was performed using the equivalent circuit previously developed for a Love mode surface acoustic wave transducer. The present model is based on the Mason equivalent circuit for inter-digital fingers. A Love mode Si02/ST -cut quartz transducer with operating frequency at 96 MHz was fabricated and the transfer function and input impedance were measured. Simulation results were compared with the experimental measurements. They showed close agreement. A. IntroductionThe previously suggested equivalent circuits for SAW (Surface Acoustic Wave) transducers were only based on blank uncoated transducers [1,4]. In this paper, simulation results of a Love mode model developed by the authors [5] will be compared with experimental measurements. A Love mode SAW transducer is a device with layers of different materials in which the propagation speed of the shear acoustic waves in the layers is smaller than that of the substrate.A Si02/ST-cut quartz structure was fabricated. The ST-cut quartz crystal was used as the piezoelectric substrate and Si02 was deposited using r.f. magnetron sputtering as the guiding layer. B. TheoryIn the equivalent circuit for inter-digital fingers, the non-piezoelectric layers were modelled by wave-guide components with no electrical coupling [5]. The Mason equivalent circuit were used for modelling the piezoelectric substrate (Fig. I). A scattering parameter (S parameter) matrix denotes the division of acoustic energy between wave-guide components and the Mason circuit. The model was implemented using the computer aided engineering tool, Libra from HP-EEsof for the analysis of the equivalent circuit. The acoustic wave propagation simulator in the layered media, developed at McGill University, Canada by E.L. Adler [6], was used both for the calculation of particle movements and the propagation speed of the acoustic wave for each inter-digital finger section. Acoustic wave energy in the inter digital finger sections were calculated using particle displacement magnitudes to acquire coefficients of the S-parameter matrix.For the Love mode acoustic wave propagation, the In-Field model [4] was used due to the direction of movement of particles. In the In-field model, the applied electric field is normal to the acoustic wave propagation direction, which is in accordance with the characteristics of Love mode acoustic waves.The acoustic energy is split and then added between the waveguide and the Mason circuit using scattering matrices. The scattering parameter matrix relates the amplitude of the incident waves on the port to those reflected from the ports. Two scattering parameter matrices were used in the input and outputs of each vertical section, respectively, A 3�port network was used in the input (shown in Fig. 2(a)).