Parameters included in coupling-of-modes equations such as coupling coefficient,
transduction coefficient and electrode capacitance are theoretically determined using
the finite-element method (FEM) for a natural single-phase unidirectional transducer
(NSPUDT) and a transduction center shift reversal of directivity transducer (TCS-RDT) on La3Ga5SiO14 piezoelectric single crystal which has a higher
electromechanical coupling coefficient compared to quartz. The electrode thickness
dependence of the parameters and the excitation characteristics of NSPUDT and TCS-RDT are investigated in detail. For NSPUDT, the parameters obtained by the present
method are compared with those obtained by the perturbation method. Furthermore,
numerical results for the frequency characteristics of NSPUDT are compared to the
experimental data, and the validity and usefulness of the present method are
demonstrated.
An equivalent network theory and a coupled mode theory are described for the analysis of excitation characteristics of interdigital transducers (IDT) for surface acoustic waves. Circuit parameters of equivalent circuits and coupling and transduction coefficients of coupled mode equations can be theoretically determined by applying the finite-element method to an infinite array corresponding to an electrically shorted or open IDT. Examples are computed for two types of IDTs fabricated on a 128° Y-X LiNbO3 substrate. One is a uniform type of IDT and the other is a single-phase unidirectional transducer utilizing internal floating electrodes.
Computed results of all the coefficients of coupled-mode equations are presented for a natural-single-phase-unidirectional transducer (NSPUDT) and an electrode-width-difference-reversal-of-directivity transducer on a 50 • Y -24 • X La 3 Ga 5 SiO 14 substrate. In order to determine all the coefficients, edge frequencies of a stop-band, standing-wave distributions at these frequencies on a substrate surface, and static capacitances of a transducer are calculated using the hybrid finite-element method. Therefore, all the effects of piezoelectric perturbation, mechanical perturbation, and energy storage due to nonradiated bulk waves are taken into account. The aluminum electrode width dependences of the coefficients are investigated in detail. Numerical results reveal that we can choose pairs of width and height of the active electrodes for a distance of λ/8 with λ being the surface acoustic wave wavelength at the center frequency between the reflection center and the transduction center. Our results of the transducer loss for NSPUDT agree well with previous experimental ones.
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