Design of ultra-wideband surface acoustic wave filters

This paper describes working principles of traditional ladder-type surface acoustic wave (SAW) filter composed by resonators. The simulation model was founded by utilizing Quite Universal Circuit Simulator (QUCS) for a seven stepped SAW filters on substrate of relaxor based ferroelectric single crystals. An extremely large wideband SAW filter of 620 MHz at 1 GHz could be obtained due to large electromechanical coupling factor (K²~60%) when the substrate is poled along (111) plane, which is presently three times larger than that achieved by conventional piezoelectric material. The band rejection could be improved by adjusting static capacitance ratio of series and parallel arm resonators. In order to optimize overall properties of the ultra-wideband SAW filter, we have modified the structure of a conventional ladder-type filter by adding a resonator to series or parallel arm to form two new notches, which sacrifices some bandwidth, but could achieve large transition band, steep transition zone and also band rejection. Finally, we discussed the influence of quality factor on the insertion loss of the SAW filters.