Interdigital Transducer Models: Their Impact on Filter Synthesis

The characterization and comparison of various interdigital transducer (IDT) models used to analyze and design SAW transversal filters are facilitated by partitioning the overall periodic transducer re- sponse into an impulse function, a discrete time sampling function, and m acoustic weighting function in order to describe both wide-band and space harmonic responses. Through this formalism the analog char- acter of an IDT is accounted for by the use of an acoustic function a(f). This perspective is applied to five interdigital transducer models: namely, impulse (equivalent circuit), impulse (sine wave), modified delta function, spectral weighting and space harmonic. The models differ by their a(r) function. The spectral weighting model is de- scribed in detail for its application to apodized and withdrawal weighted transducer design. A new modified delta function model is shown to agree qualitatively with the spectral weighting model for single electrodes, Aperture weighting laws are derived for the models using sampling theory; it is shown that the cla~sicf;~/~ frequency de- pendence holds only for the narrow-band case f = fn and that, in gen- eral, compensation is required for the acoustic weighting function of the model used. Guidelines for evaluating whether or not compensa- tion is needed as well as compensation techniques and alternatives are described.