A method for analysis of bulk mode effects on SPUDT based filters

Single Phase Unidirectional Transducer (SPUDT) based SAW filters are realized by withdrawal weighting of the sources to eliminate apodization losses. The withdrawal weighting creates unwanted bulk mode responses which degrade the filter rejection. For the rotated YX quartz substrate, the most commonly used substrate for this type of filter, the fast shear (FS) and longitudinal (L) modes contribute most of the degradation. This paper presents a method of analysis of SPUDT filters using a COM analysis with 2N acoustic ports to account for N propagating modes. This analysis with N=3 (SAW+FS+L) is used to predict the degradation in the rejection for a 188 MHz filter for PCN applications. An approximation is made that both the FS and L modes are surface propagating modes. Comparison of analysis and measurement is given, showing very good agreement