Compact microstrip bandpass filter with sharp roll-off and broad stopband using modified 0° feed structure

Abstract

A compact microstrip bandpass filter (BPF) with sharp roll-off skirt and broad stopband, consisting of the proposed modified 0° feed structure and the shorted stub-loaded tri-section stepped impedance resonator (SSLTSIR), is presented. The first two high-order parasitic resonant frequencies of the conventional 0° feed structure can be suppressed when a pair of open-circuited SIRs is loaded on its upper transmission path. Thus, the proposed modified 0° feed structure with broad stopband response is realized. Moreover, the theoretical analysis and numerical investigations are carried out. Furthermore, by embedding the SSLTSIR into the proposed modified 0° feed structure, a BPF with sharp roll-off skirt and broad upper stopband is achieved. A demonstrated BPF is fabricated and measured, and its measured results agree well with the simulated ones.