High selectivity wideband bandpass filters based on flexibly transferring the structure of a coupled-line

Abstract

Two novel high selectivity wideband bandpass filters (BPFs) based on flexibly transferring the structure of a coupled-line are proposed in this paper. More flexible design and more widely used scenarios are obtained due to the transformational structure of a coupled-line. An open coupled-line and a transmission line connecting the input port bring the wide bandwidth and the great frequency selectivity to BPF Ⅰ. The proposed BPF Ⅱ realizes two transmission zeros close to the passband by a stepped-impedance stub. The generalized theoretical analysis of the two asymmetric BPFs is presented. Two five-pole BPFs both working in the 3.5-GHz band with two transmission zeros on each side of the passband are simulated, fabricated, and measured to demonstrate the feasibility of the design. The 3-dB fractional bandwidths (FBWs) of the two BPFS are 73.17% and 78.34%, separately. Good consistencies are observed between the simulated and measured results.