Institute of physics conference on acoustic surface waves and their applications

Abstract

This chapter presents some of the second-order effects that must be dealt with in the design of precision SAW filters with certain specifications. Their reduction is generally accompanied by some type of trade-off between insertion loss, fractional bandwidth, and close-in sidelobe suppression. The most troublesome second-order problem is due to electromagnetic feedthrough, especially at ultra-high frequencies or above. This can only be minimized by critical attention to packaging, shielding, and the proper placement of ground lead connections. The trade-offs in linear phase SAW filter design should include factors such as (a) finger-reflections, (b) bulk waves, (c) diffraction, (d) beam steering, (e) acoustic attenuation and (f) electromagnetic feedthrough. Some finger reflection interference can be reduced by the use of interdigital transducers (IDTs) with split-electrode geometries, while bulk wave interference can be circumvented in some designs by the use of a multistrip coupler. All the IDTs that emit surface waves will also emit bulk waves to some extent. As bulk waves have higher velocities than the surface waves, interference between bulk waves and SAW at the receiving IDT will be most pronounced at the high-frequency end of the filter passband. Two approaches can be used to relate the SAW diffraction pattern of a radiating IDT. One way is to consider that each aperture corresponds to the overlap region of a finger pair. The other is to identify diffraction apertures with individual finger lengths.