4 - Equivalent Circuit Models for a SAW Filter

Abstract

This chapter outlines the mathematical relations for three important equivalent circuit models used in SAW filter design. These are (1) the delta function model, (2) the crossed-field model, and (3) the impulse response model. Each model reveals an important feature for SAW filter design. The delta function model is the simplest of the various models that describe the performance of a bidirectional SAW interdigital transducer (IDT). It relates the restriction placed on apodizing both IDTs in a SAW filter. The crossed-field model is derived from the Mason equivalent circuit employed for modeling acoustic bulk wave piezoelectric devices. It yields information on IDT input and output impedance level, and on triple-transit interference levels in the filter passband. The impulse response model also provides information on input and output impedance levels, as well as on the frequency sensitivity of excited IDT electrodes. It also provides insight into the frequency scaling that must be employed in the determination of apodization quantities for broadband or chirp SAW filters. The chapter also deals with the inherent power loss of bidirectional IDTs as well as degradation imposed by triple-transit-interference and electromagnetic feed through on power transfer characteristics.