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A novel ultra-wideband (UWB) bandpass filter is proposed using a multiple-mode slotline resonator and back-to-back microstrip-slotline transition. A stepped-impedance slotline resonator is constructed on the ground plane with its first three resonances occurring within the targeted UWB passband. Two microstrip feed lines are placed on the opposite plane, perpendicular to the slotline resonator, thus making up a hybrid microstrip-slotline UWB bandpass filter in the format of back-to-back transition. As its geometry is properly rearranged a compact UWB filter with improved out-of-band performances is designed. Measured results exhibit an UWB passband from 2.95 to 10.16 GHz with mid-band insertion loss of 1.27 dB and group delay variation < 0.41 ns.