A global optimization procedure for SPUDT filters

Published design methods of single phase unidirectional (SPUDT) filters using internal reflections only allow for individual transducer optimization:electroacoustic transfer function (amplitude and phase), and, acoustic reflection of the loaded transducer. Consequently, multiple reflections between transducers are not taken into account for passband ripple optimization. We present a generalized method which optimizes directly the amplitude and group delay of the entire filter. The new method takes into account inter-transducer reflections, while controlling simultaneously each transducer. Furthermore, the reflection weighting functions of grating shields needed between transducers to obtain low feed through level are also taken into account in this new algorithm. Filtering characteristics as well as size reduction of various low loss bandpass filters have been dramatically improved using this new global optimization procedure : we present various SPUDT based filters designed using this algorithm. The size of a 71 MHz filter for mobile GSM was greatly reduced while keeping a two SPUDT structure: this filter fits in a SIP 5 package while the previous filter required a DIP 14 package