Design of withdrawal-weighted SAW filters

This paper presents a new design algorithm for a withdrawal-weighted surface acoustic wave (SAW) transversal filter. The proposed algorithm is based on the effective transmission loss theory and a delta function model of a SAW transversal filter. The design process consists of three steps, which eventually determine eight geometrical design parameters for the filter in order to satisfy given performance specifications. First, the number of fingers in the input and output interdigital transducers (IDTs), plus their geometrical sizes is determined using the insertion loss specification. Second, the number and positions of the polarity reverses in the output IDT are determined using the bandwidth and ripple specifications. Third, the number and position for withdrawing and switching specific fingers in the output IDT and attached electrode area are determined to achieve the desired sidelobe level. The efficiency of the technique is illustrated using a sample design of an IF filter consisting of a uniform input IDT and withdrawal-weighted output IDT. The proposed algorithm is distinct from conventional techniques in that it can optimize the structural geometry of a withdrawal-weighted SAW filter in a direct manner by considering all the performance specifications simultaneously.