Balanced BPF with wide upper stopband and wide common-mode suppression response for WiMax applications

This article presents a new balanced bandpass filter (BPF), which is designed using coupled stepped-impedance slot-line resonators (SISLRs) fed by microstrip feeding lines. The SISLRs are deployed symmetrically with respect to the plane of symmetry to achieve favorable differential-mode (DM) response and high common-mode (CM) rejection. For DM operation, the plane of symmetry is virtually grounded, and resonance can occur for each half SISLR. For CM operation, the resonator boundary along the plane of symmetry is open-circuited, rendering the half SISLRs structure incomplete and hence, causing weak signal transmission. A prototype of the proposed balanced BPF was implemented, and good agreement was observed between simulated and measured results. The designed BPF has a measured minimum DM insertion loss of 1.13 dB in the passband of 3.38-3.72 GHz, in which the measured CM suppression is larger than 55.76 dB. In particular, the upper DM stopband with 20 dB rejection extends up to 21.05 GHz, and the CM suppression is larger than 26.5 dB in the whole displayed frequency range of 2-22 GHz.