Design of Balanced Wideband BPF Based on Tri-Mode Slotline Resonators

In this brief, a tri-mode slotline resonator (TMSR) is designed. The proposed resonator has similar performances to those of the conventional microstrip multimode resonator (MMR), while it is located at the bottom ground layer. Meanwhile, the method of characteristic mode analysis (CMA) is innovatively applied to the design process of TMSR, and specific formula results can be obtained. By properly adjusting the dimensions of the proposed TMSR, three resonant modes can be controlled easily, so that a broad differential-mode (DM) transmission response can be realized and the DM stopband can be up to 17.5 GHz (
$3.04f_{0}$
). By employing balanced microstrip/slotline transition structures, a high and wideband common-mode (CM) suppression can also be achieved, which is independent of the DM responses. Hence, only the DM performances need to be focused, which simplifies the design procedure significantly. In order to suppress the special electromagnetic interference, a notched band structure is added without increasing the size of the filter. Moreover, three-slotline couple structures and enhanced coupled structures are discussed to further optimize the filter performances. To validate the design concept, one prototype is implemented following the given design procedure. Good agreement between the measurement and simulation results can be observed.