Design of high selectivity DART SPUDT filters on quartz and lithium tantalate

The design of highly selective moderate insertion loss SAW SPUDT filters with flat group delay and low time spurious response is quite challenging. Both the transduction and reflection functions must be spatially varying and accurately designed and implemented to achieve good shape factors along with low triple transit spurs. We present an approximate solution to the coupled mode equations describing a SAW transducer which is valid for both spatially varying transduction and reflection. Next we show how to invert this approximate solution to design the required transaction and reflection functions. The actual implementation is a weighted DART structure in which we obtain continuous spatial control over the reflectivity by varying the reflector dot density in the DART. The physical implementation has a significant impact on performance. We explore filters with apodized, apodized/withdrawal and withdrawal weighted transducers. We also investigate weighting only one transducer as well as both in order to obtain the best performance in the presence of second order effects such as diffraction. Experimental results for flat topped sharp cut-off filters on both quartz and 112° lithium tantalate are presented The quartz devices are 1 MHz and 2.5 MHz bandpass filters centered near 180 MHz. The 2.5 MHz filter has a shape factor better than 1.3:1, exhibits less than 30 nsec of group delay variation across the passband and possesses less than 14 dB insertion loss. The lithium tantalate device has a 5 MHz passband centered near 185 MHz and exhibits similar performance, 40 nsec of group delay variation, better than a 1.4:1 shape factor and less than 12.5 dB insertion loss