Balanced absorptive coupled-line dual-band bandstop filter with inherent dual-band common-mode noise suppressions

In this paper, a planar balanced absorptive dual-band bandstop filter (BSF) with inherent common-mode (CM) noise suppressions is proposed. The symmetrical structure consists of four pairs of coupled-lines, with two pairs forming stepped-impedance resonators and the remaining two pairs serving as stubs. Resistors are inserted between coupled-lines to absorb reflected signals. Under CM excitation, the BSF achieves stopbands at 1.7 and 3.62 GHz with 3-dB bandwidths of 23.5% and 10.8%. Under differential-mode (DM) excitation, the reflection coefficient is below -9 dB. The circuit with a size of 58.8 mm × 28.2 mm is fabricated and measured to verify the design.