Scattering Parameters of Interdigital Surface Acoustic Wave Transducers

Abstract

Abslracf-A simple physical model is proposed to obtain the scattering parameters of an interdigital surface acoustic wave transducer. The model is useful in predicting the transient response of the scattering parameters. Experimental measurements of the transient response are found to be in good agreement with theory. Scattering parameters of apodized transducers and of transducers in which surface waves are incident on both the acoustic ports are calculated. The model does not take into account impedance discontinuities produced by the transducer electrodes, but apart from this limitation is sufficiently general and should be applicable to a wide range of electroacoustic transducers. HEN A surface acoustic wave is incident on an electrically loaded interdigital transducer (IDT), a part of the wave energy is absorbed, a part is transmitted forward, and a part is reflected. The IDT can thus be considered as a scatterer of surface waves. A knowledge of the scattering parameters is obviously important in the design of surface wave devices. The scattering parameters of the IDT have been calculated as a function of the electrical load by Smith et al. [l] using an equivalent circuit model. Their method however needs laborious calculations and does not give any physical insight into the problem. The purpose of this paper is to present a simple physical model which can be readily used to obtain the scattering parameters. The basis of the model is the realization of the fact that the voltage developed across the receiving transducer by the incident wave is responsible for the regeneration of acoustic waves by the receiving transducer. The net acoustic field in the medium is then the sum of the incident and the regenerated fields. This approach readily leads to simple formulae for the scattering coefficients. The scattering coefficients obtained by this approach are found to be in agreement with those obtained by Smith ef al. The utility of the model however extends much beyond the mere calculation of scattering parameters at the resonant frequency. The transient response of the scattering parameters can be calculated from the model. The predictions of the model have been experimentally verified by studying the transient response of the reflection and transmission coefficients of the IDT. Knowledge about the transient response enables one to calculate the maximum fractional bandwidth of a surface wave reflector and of a unidirectional transducer using such a reflector. Other applications of the model include the calculation of scattering parameters when i) acoustic waves are incident simultaneously on both ports of the IDT, and ii) the width of the incident acoustic beam is different than the aperture of the transducer. The latter situation is important in the study of apodized transducers. The model proposed here is sufficiently general and should be applicable to any electroacoustic transducer of either bulk or surface acoustic waves.