A Differential-Mode Wideband Bandpass Filter on Slotline Multi-Mode Resonator With Controllable Bandwidth

This letter presents a novel differential-mode (DM) wideband bandpass filter (BPF) on slotline multi-mode resonator (MMR) with controllable bandwidth. The BPF consists of a cross-shaped slotline MMR etched on the ground plane and two pairs of microstrip feeding line. On the one hand, with proper placement of the slotline MMR, the coupling between the slotline MMR and DM feeding lines can be flexibly controlled to achieve the desired DM passband. On the other hand, the cross-shaped slotline MMR is fully broken or disconnected in the middle portion under the CM excitation, thereby obtaining the CM suppression over the DM passband. Moreover, the bandwidth of the DM passband can be freely adjusted through the impedance ratio of the slotline MMR. To verify the design concept, a filter sample has been designed and fabricated. Within the DM passband, the insertion loss of the CM signals is higher than 20.0 dB, the 3 dB fractional bandwidth of the DM passband achieves around 107.7%. The measured results are obtained in good agreement with the predicted ones.