Balanced Dual-Band BPF and FPD Using Quad-Mode RLR With Improved Selectivity

In this brief, a balanced dual-band bandpass filter (BPF) using quad-mode ring loaded resonator (RLR) is presented. The resonance characteristics of the proposed quad-mode RLR are analyzed and the design graphs of the electrical lengths and the four resonant modes are given. Based on the above analysis, four resonant modes are applied to achieve two differential-mode (DM) passbands. U-shaped microstrip lines and stepped impedance microstrip/slotline (MS) transition structures are employed to achieve good common-mode (CM) rejection without affecting DM ones. In addition, an additional transmission pole is realized in the lower passband due to the stepped impedance slotlines. Meanwhile, six transmission zeros (TZs) are generated due to the horizontal transmission interference of the resonator and new coupling path, which improves the selectivities of the passbands. Furthermore, based on the proposed filter, a balanced dual-band filtering power divider (FPD) is designed for miniaturization. By loading isolation resistors on the slotline, the output ports isolation is improved. To verify the theoretical design, two proposed designs have been manufactured and measured. The measured and simulated results are presented with a good agreement.