Planar Balanced Filters with Ultrawide Band Common-mode Suppression and Tunable Notch

An ultra-wideband differential filter with common-mode suppression over the whole band is presented. To this end, a pair of back-to-back balanced microstrip-to-slotline transitions are utilized. It is demonstrated that due to the electric wall at the symmetry plane of the structure, which is only available for differential excitation, strong coupling, and, as a result, a UWB bandpass behavior for differential signals is achieved. In contrast, the structure strongly rejects common-mode signals. It is further shown that different types of unbalanced filters can be integrated with the proposed structure, thus achieving elaborate frequency responses while avoiding the complexities associated with balanced filter design.