Design of narrow bandwidth elliptic-type SAW/BAW filters

The passband width obtained in the SAW/BAW filter is inherently limited by the electromechanical coupling factor K² of piezoelectric substrate. Proposed is a new design technology to flexibly control the passband width without sacrificing insertion loss, out-of-band rejection and steepness of the transition bands. A bandpass LC filter has been designed based on the traditional elliptic filter design. The LC resonators are replaced with SAW/BAW resonators in this filter and the design of the resonators is optimised to suppress passband ripples generated by finite capacitance ratio γ. It revealed that filters with different relative bandwidth are easy to design using one piezoelectric substrate. This technique is applied to the design of a SAW filter fabricated on the Cu-grating/15°YX-LiNbO₃ structure.