Overview of design challenges for single phase unidirectional SAW filters

A comprehensive discussion of the design synthesis challenges for single-phase unidirectional transducers (SPUDTs) is presented. The complexity of SPUDT filter design results from the fact that a SPUDT deliberately includes reflections internal to the transducer to cancel the effects of regeneration reflection. This destroys the validity of the impulse-response model which is the basis for all conventional SAW (surface acoustic wave) transducer designs. Although a recent generalization of the impulse response model includes effects of uniform internal reflections, it is of limited usefulness, because a high-performance SPUDT filter requires both weighted reflection and weighted transduction. Presented is a six-step SPUDT synthesis procedure which includes a new method for determining transduction and reflection weighting functions that satisfy a desired amplitude and phase response, achieve low insertion loss, achieve low triple transit, allow use of simple matching networks, and properly account for the interactions between transduction and reflection effects under matched conditions. Fundamental insertion-loss limitations for SPUDT filters are discussed and areas for future SPUDT research are outlined. Good experimental performance is demonstrated for a bandpass filter that uses a new SPUDT structure, the EWC/SPUDT, the geometry of which is also shown.<>