P6G-2 Synthesis of Frequency Response for Wideband SAW Ladder Type Filters

Numerical synthesis of frequency response is discussed for a ladder type filter consisting of ultra-wideband SAW resonators on Cu-grating/15degYX-LiNbO₃ structure. For the synthesis, two parameters of each resonator, i.e., the static capacitance and resonant frequency were numerically optimised, while the capacitance ratio and figure of merit were fixed at specific values determined by substrates and grating electrodes employed. First, six-stage SAW ladder type filters having the passband width of 7 to 21% were designed. The result showed that the specified bandwidth is successfully synthesised. Next, a six-stage ladder type filter having the passband width of 8% was fabricated on Cu-grating/15degYX-LiNbO₃ structure for experimental discussion. It was shown that the measured frequency response of the filter was in good agreement with the design, where some in-band spurious dips were removed by weighted dummy electrodes. By making analysis on the behaviour of resonant and anti-resonant frequencies of each resonator, it is shown that the method can be extended to the synthesis of filters having sophisticated frequency responses. The result suggested that the proposed method could become one of the powerful design tools for SAW ladder type filters with specified passband width.