Synthesis of Dual-Wideband Bandpass Filters With Source-Load Coupling Network

A class of dual-band bandpass filters (BPFs) have been exactly synthesized and practically designed by an equivalent circuit formed by a modified transversal array and its related filtering function. Either open- or short-circuited stepped-impedance resonators (SIRs) are mutually coupled as the basic filter block. By the Richard´s transformation, the even-/odd-mode bisection circuits are made equal to their corresponding LC circuits. Combining these two parts together, an equivalent circuit of the proposed dual-band filter is obtained, which consists of a transversal structure and a Π-shaped inductor/capacitor network. With the addition of this Π-shaped network as the source-load coupling circuit, transmission zeros (TZs) can be generated on both sides of the desired passband to further improve the out-of-band rejection. To model and synthesize this class of equivalent circuit, a generic filtering function is derived by introducing a new filtering design parameter ε_o, namely, the out-of-band rejection factor. With the use of ε_o, the filtering function is modified to have a controllable out-of-band rejection while the bandwidth and TZ location of the filter are fixed. Two filter prototypes with open- and short-circuited SIRs are synthesized, designed, and fabricated in the last part to have supported both the proposed filter structure and the filtering design principle.