Design of microstrip tri-mode balun bandpass filter with high selectivity

A new microstrip tri-mode balun bandpass filter (BPF) with good filter-type and balun-type functions is presented. To realise the tri-mode BPF performance, the multimode stepped-impedance resonator is introduced and parallel coupled to both the balanced and the unbalanced ports. To achieve 180° phase difference, two types of transition structures, i.e. the microstrip–coplanar waveguide–microstrip transition and the microstrip–coplanar stripline–microstrip transition, are adopted for the two balanced output ports design, respectively. A balun BPF operating at 2.6 GHz is designed and fabricated to validate the feasibility of the new approach. Results indicate that the proposed balun BPF exhibits not only high selectivity with two transmission zeros locating at 1.7 and 3.7 GHz but also good balance performance with 0.5 dB magnitude imbalance and 5° phase imbalance between the balanced outputs.