Optimization of modified Hanma-Hunsinger cell geometry for the design of high performance SAW filters

The design of high performance SAW filters requires accurate prediction of second order effects such as diffraction, electroacoustic coupling, electrical parasitics, and bulk mode signals. Filters comprised of bidirectional transducers having 3 or 4 strips per wavelength can be designed to achieve passband ripples of <; 0.1dB-pp and <;0.5deg-pp (with triple transit removed), and close-in rejection better than 60dBc. The drawback of these structures is the triple transit time spurious which degrades the passband ripples, increasing with lower insertion loss. The triple transit can be reduced by 15dB or more by adding mechanical reflectors in the transducers at the expense of reduced accuracy of the analysis. The reflectors can be added to the transducers by replacing groups of strips or cells with modified Hanma-Hunsinger cells. However, care must be taken to ensure that the localized electroacoustic coupling and phase delay through the cells are not changed. Small changes in these parameters can severely degrade the filter´s characteristics. Analysis of diffraction effects of transducers with mechanical reflection is not straightforward and not as accurate as for bidirectional transducers. The goal of this study is to find the optimum geometry of modified Hanma-Hunsinger cells which will have nearly identical electroacoustic coupling in the forward direction and phase delay through the cell. These two conditions will allow the filter design to be optimized with bidirectional transducer analysis tools. A final analysis can then be made of the modified transducers to determine the reduction in triple transit. The optimum geometry for the cells is determined from analysis using the original coupled FEM/BIE numerical model of P. Ventura presented at the 2013 IEEE Ultrasonics Symposium. Comparison of key cell parameters for bidirectional cells and modified Hanma-Hunsinger cells will be presented. The impact of partially buried electrodes (transverse mode suppression) and raised electrodes (frequency trimming) on the design will also be examined. Measurements on a filter designed with the above method will be compared with analysis.