Design of a Compact Dual-Band Bandpass Filter Using Coupled Stepped-Impedance Resonators

A compact dual-band bandpass filter (BPF) using coupled three-section stepped-impedance resonators (SIRs) has been proposed and analyzed in this letter. By its symmetrical merit, the even-odd mode analysis method is used to obtain the input admittance of its bisection circuits. After the Richard´s transformation, each bisection circuit is derived as an equivalent LC circuit. Combining these two LC circuits, the equivalent circuit for the proposed filter is built with its design parameters as a function of even-/odd-mode impedances of the coupled SIR. The proposed circuit is then classified into two distinctive parts, a transversal structure for the first two resonances and a Π-shaped inductor network for the mutual coupling between the source/load. Thus, there are two reflection zeros appearing in each passband and five transmission zeros (TZs) generated to improve the out-of-band rejection. A prototype dual-band filter has been designed and fabricated to verify the predicted filtering performance and the accuracy of the presented design principle.