High Selectivity Dual-Band Balanced BPF With Controllable Passbands Based on Magnetically Coupled Capacitor-Loaded SIRs

In this brief, a novel capacitor-loaded stepped-impedance resonators (CL-SIRs) is proposed and adopted to construct dual-band balanced bandpass filter (BPF) with controllable passbands. The differential mode (DM) and common mode (CM) characteristics of the CL-SIRs are discussed in detail, and two DM resonant frequencies that can be independently controlled are selected to design the desired dual DM passbands. A novel dual-frequency feeding structure is proposed to not only independently control the external quality factors of the two DM passbands but also suppress the unwanted harmonics of DM bisection. Furthermore, a intrinsic high CM suppression is achieved by utilizing the magnetically coupled scheme. Also, multiple transmission zeros are generated because of the introduced source-loaded coupling, which highly improved the selectivity of two DM passbands. For verification, a dual-band balanced BPF operated at 2.5 GHz and 3.5 GHz is designed and fabricated. The measured results agree well with the simulated results, and the filter exhibits the attractive features of design flexibility, wide-stopband, high selectivity, and favorable CM suppression.