A Simple Design Procedure for Triple Transit Suppression in an Apodized - Withdrawal Weighted Transducer Filter Structure

摘要

High performance SAW filters are characterized by their linear phase, low shape factor, small passband ripple, and high rejection. Classical filter design structures can achieve these performances, however the level of triple transit time spurious for these structures is typically twice the insertion loss + 6dB. The triple transit level can be reduced by adding low loss cell structures (such as Hanma-Hunsinger, EWC) to the transducers, however the filter analysis becomes inaccurate with the introduction of mechanical reflections (problems analyzing diffraction effects). The objective of this work is to develop a synthesis technique for high performance SAW filters with triple transit suppression. A synthesis technique has been developed for inline filters on quartz using an apodized transducer and withdrawal weighted transducer (WWT) with split fingers. The first step of the synthesis is to design the apodized and WWT using a classical analysis which includes secondary effects including complete diffraction analysis and distributed bus bar parasitics. Next, an analysis of a WWT-WWT structure is made using identical WWTs with the classical analysis. Then a low loss synthesis is made with a pair of withdrawal weighted DART transducers (WWDT) obtained by replacing some of the split finger cells with modified Hanma-Hunsinger (HH) cells. The synthesis optimizes the triple transit (TT) for the WWDT-WWDT structure while keeping S21 nearly the same as for the WWT-WWT classical analysis (TT gated). The final filter design is an inline filter with an apodized and a withdrawal weighted DART transducer. The synthesis technique was used to design a 23dB loss 180MHz quartz filter with 0.5dB bandwidth of 2.57MHz and a 50dB bandwidth of 6.11MHz. Devices were manufactured with both the classical WWT design and the version with HH-cells. Measured data is presented for both versions. The classical design achieved 51dB TT while the version with HH-cells achieved better than 62dB. The developed synthesis technique for improving triple transit in high performance SAW filters suppresses the triple transit level by more than l0dB while retaining other classical filter performances.