A Planar Multiband Balanced Bandstop Filter

This paper introduces for the first time, a novel architecture of differential (balanced) bandstop filter (BSF), which can realize an arbitrary number of stopbands with arbitrary order. The design incorporates in-series cascade of K N-band sections in a branch line structure to realize a K-th order N-band balanced BSF transfer function. The common-mode response is controlled by introducing open stubs to the symmetry plane of the design. For demonstration, a second order dualband differential BSF with center frequencies 1.15GHz and 1.7GHz is designed and fabricated. The simulation results and the measured results show a good agreement with each other.