Design of narrow bandwidth ladder-type filters with sharp transition bands using mutually connected resonator elements

This paper proposes a new design technique for a ladder-type filter to reduce the passband width without sacrificing the insertion loss, out-of-band rejection and steepness of the transition bands. First, it is shown that two transmission zeros can be generated by connecting an additional resonator in either series or parallel arm of the ladder filter topology. This new filter topology can be designed systematically by the derived-m transformation. Secondly, it is demonstrated that the narrow bandwidth, sharp transition bands and large out-of-band rejection are simultaneously realised by applying the present technique to specifically designed ladder-type filters. Finally, this technique is applied to the design of a SAW filter fabricated on the Cu-grating/15degYX-LiNbO₃ structure.