Strip-Loaded Slotline Resonators for Differential Wideband Bandpass Filters With Intrinsic Common-Mode Rejection

In this paper, novel strip-loaded slotline resonators are proposed for design and exploration of differential wideband bandpass filters (BPFs) with intrinsic common-mode (CM) rejection. When the loaded stubs on the slotline resonator are in the format of microstrip lines, there are two types of lines in the resonator and the resonator can be fed or tightly coupled from the loaded strip stubs. In this way, the troublesome orthogonal transitions for feeding the slotline resonator can be avoided. With the use of this hybrid microstrip/slotline structure, the proposed stub-loaded slotline resonators have their CM rejection themselves as an inherent property. Only the differential-mode (DM) transmission performances need to be focused on. In this context, the constructed resonators operate under triple-and quadruple-mode operation and they are tightly coupled with feeding lines via parallel coupled lines. After the multimode resonance properties of these proposed hybrid resonators are investigated, two differential BPFs with five and six in-band transmission poles are designed, fabricated, and measured. All the simulated frequency responses of these two designed filters have demonstrated their excellent DM wide passband performance with high CM rejection, as validated in experiment.