Wide band tapered SAW filters with improved shape factor and phase response

The design technique of IF filters based on tapered SPUDT technology is widely used for new picocell base stations market. The performance of these SAW filters (insertion loss, bandwidth and transition band) depends greatly on the coupling of the substrate, SPUDT cell parameters, and on the tapered transducer design (transduction and reflectivity weighting, weighting implementation and transducer periodicity as a function of position across the aperture). The electrode widths in these structures vary significantly both across the aperture and in wave propagation direction. This poses an additional challenge to the fabrication process. The problem is magnified by increasingly stringent specifications for base station filters, including wider bandwidths and higher selectivity. In the present work the effects of the fabrication process used in manufacturing of these devices was considered. A discrepancy was found between simulated and measured device responses due to the deviation of fabricated electrode width from that specified by the transducer design. The value of this deviation was found to be a function of the electrode width and local periodicity. By correcting this deviation in the layout, the agreement between the simulation and measurements was improved. Results of a 400 MHz with 17% bandwidth tapered SPUDT SAW filter with and without the layout correction are presented in this paper.