Tri-band bandpass filter designed by a fast hybrid approach

Multiband bandpass filter is a key component in multistandard wireless communication system. This paper proposes a novel tri-band bandpass filter (TB-BPF) which consists of two mesh-type multimode resonators (MT-MMR) and two stub-loaded U-type feedlines. The MT-MMR is formed by parallel connecting two open-end stubs and one mesh-type structure. Such specific configuration is featured by four inherent transmission zeros. The in-depth analysis on the generation mechanism of the transmission zeros is made, and then a fast hybrid approach is developed for design procedure. Some explicit design equations are presented for fast determining initial values of some key parameters, and then further optimized by using a constraint condition, which effectively accelerate design process. For verification, an example of the TB-BPF was designed fabricated and measured. Its center frequencies are at 3.16, 4.27, and 5.70 GHz, respectively. Six transmission zeros are generated for sharp frequency selectivity, and are at 1.46, 3.58, 3.86, 4.73, 5.51, and 7.18 GHz, respectively. Its size is about 0.59 x 0.19 lambda(2)(g).