Selectivity-Enhancement Technique for Stepped-Impedance-Resonator Dual-Passband Filters

A class of stepped-impedance resonator (SIR) dual-band bandpass filter (BPF) with selectivity-enhancement cells is reported. These dual-band filtering cells, which are composed of two open-ended transmission-line segments interconnected at the input, are added at one node of the input and output coupled-line stages. Thus, with regard to the SIR dual-band BPF without cells, they increase the order of each passband by two and simultaneously generate transmission zeros (TZs) at both sides of the two transmission bands. The operating principles of the proposed dual-band BPF are detailed at the coupling-routing-diagram level and design guidelines for its transmission-line configuration are provided. Moreover, for experimental-validation purposes, a 1.57-GHz/2.38-GHz dual-band BPF microstrip prototype with symmetrical absolute bandwidths is manufactured and characterized.