High-selectivity wideband bandpass filter based on quintuple-mode stub-loaded resonator and defected ground structures

A high-selectivity wideband bandpass filter (BPF) based on a quintuple-mode stub-loaded resonator and defected ground structures (DGSs) is presented. Thanks to the symmetrical topology, the even/odd theory is adopted for analyses. By properly adjusting the stub dimensions, the resonant frequencies of the transmission poles and zeros can be controlled to obtain wide passband and sharp out-of-band rejection performances. The bandwidth of the upper stopband is further extended by introducing a pair of DGSs under the interdigital coupled line feeding structures, and the spurious response rejection is improved by 18.9 dB. A BPF prototype that operates at 6 GHz with a fractional bandwidth (RBW) of 70 % is manufactured for validation. The measured insertion loss and return loss are 1.3 and 17 dB, respectively. The measured out-of-band rejection over DC-2 GHz and 8.8-21 GHz are better than -30 dB.