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A novel design of a differential-mode (DM) wideband bandpass filter (BPF) with enhanced common-mode (CM) suppression using a slotline resonator is presented. The BPF consists of two pairs of parallel microstrip transmission lines, a pair of transverse microstrip sections connected to the transmission lines, and a longitudinal slotline resonator placed in the ground plane. With proper placement of the slotline resonator, the mode coupling between the slotline mode and the (DM) signals can be maximized, while that between the slotline mode and the CM signals can be minimized. It is found that throughout the (DM) passband, the insertion loss of the CM signals is higher than 47.5 dB, the fractional bandwidth defined by the 10 dB return loss for the (DM) signals is about 105%, and the insertion loss for the (DM) signals ranges from 1.22 to 5.82 dB over the fractional bandwidth. Good agreement between simulated and measured results is obtained.