Design of compact tri-band bandpass filter using stub-loaded quarter-wavelength SIRs

Abstract

A compact tri-band bandpass filter (BPF) with high selectivity and wide stopband performance is developed based on a modified stub-loaded quarter-wavelength stepped-impedance resonator (SL-SIR). The modified SL-SIR has flexibly controllable resonance modes under different schematics of the loaded stubs, from which, a tri-mode case is chosen to design a tri-band BPF with controllable centre frequencies and bandwidths. Furthermore, up to ten transmission zeros are created by introducing two source-load couplings simultaneously, which not only enhance significantly the selectivity of three passbands, but also widen greatly the stopband of the BPF. The tri-band BPF, operating at 1.27, 3.65, and 5.20 GHz, is designed and fabricated, and its measured responses are in good agreement with the simulated ones.