Investigations on radiation and reflection characteristics of SAW interdigital transducers by equivalent circuit model.

Abstract

The admittance parameters Y_&ltij&gt of the uniform types of the single and double IDT (Interdigital Transducer) are derived in closed form. In these derivations, the image parameter theory is used, and the reflections due to the discontinuity of acoustic wave impedances between the electroded and nonelectroded regions are taken into account. By these simple expressions of Y_&ltij&gt, the driving point admittance Y^&lt(E)&gt of IDT and the reflection coefficient Γ_R of IDT as a reflector are derived in closed form. Next, many numerical calculations and experiments about Y^&lt(E)&gt and Γ_R of both types of IDT are performed with LiNbO_3 in order to investigate their characteristics. In particular, Γ_R of the IDT reflectors are presented under some kinds of electrical loads. From these theoretical and experimental investigations the followings are cleared : (1) The differences of characteristics for Y^&lt(E)&gt and Γ_R between the single and double electrodes escalate conspicuously according as finger pairs increase. (2) The theoretical values obtained here about the double electrode agree with those obtained from the model neglecting the second order effects. (3) The effective impedance discontinuity factor τ_e_f_f evaluated from the experiments on Y^&lt(E)&gt is approximated by the velocity discontinuity factor τ_v, while τ_e_f_f from the experiments on Γ_R is somewhat less than τ_v. (4) IDT reflectors having large |Γ_R|(&gt0. 9) are obtained easily by small number of finger pairs (N≦30).