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A compact differential ultra-wideband (UWB) bandpass filter (BPF) with planar structure is presented. The circuit of the proposed filter is analyzed, showing that the differential-mode circuit, which consists of two short-ended stubs and an open-ended parallel-coupled lines, performs as a UWB BPF. While the common-mode circuit performs as a UWB bandstop filter, and the common-mode responses are suppressed across the whole UWB differential passband. A microstrip differential UWB BPF is designed, simulated and measured. Simulated and measured results show that the proposed filter has compact size (0.35λg×0.7λg), UWB differential passband (3 dB fractional bandwidth 119%) and UWB common-mode suppression (-10 db attenuation across 2.6-12 GHz).