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 Pi-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.