3 - Principles of Linear Phase SAW Filter Design

Abstract

This chapter discusses the principles of linear phase SAW filter design. Filters with linear phase response are employed in communications circuits when distortion of the processed signal is to be avoided. A linear phase filter is also termed as non-dispersive one. In such a filter, both the phase velocity and group velocity are ideally constant and equal over the desired frequency band of the filter. All of the frequency components of an input signal experience the same time delay and attenuation in passing through the filter so that it emerges in undistorted form. The conventional “linear phase” passive L-C filters have some inherent degree of phase nonlinearity. The degree to which their linearity of phase response is achieved over a prescribed frequency range increases with the order of the filter. The resultant overall size of a passive L-C filter, together with its complexity and cost, may render it unsuitable for many applications, especially mobile or airborne ones. The chapter also outlines the delta function model. The delta function model provides basic information on the transfer function response of a SAW filter. The chapter presents the delta function model as a basic design tool for the preliminary determinations of the frequency response of SAW filters with bidirectional interdigital transducers (IDTs). The delta function model approximates the complex electric field distribution between the adjacent fingers of an excited IDT as a discrete number of delta function sources.