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A photonic approach to polarity-switchable ultra-wideband (UWB) pulse generation is proposed. The proposed setup principally consists of an electrooptic phase modulator (EOPM) and a tunable optical comb filter constructed with another EOPM inside a Sagnac interferometer. The comb filter working at its linear region performs the phase-to-intensity conversion to generate UWB pulses with monocycle shape. By adjusting the bias voltage of the EOPM inside the Sagnac interferometer, a shift of the comb filter can be achieved, which is the key point for a pulse polarity switch. A pair of monocycle pulses with reversed polarities was experimentally obtained by applying the dc bias of 0 and 1.9 V, respectively. The convenience to electrically control the pulse polarity is favorable for biphase modulation in UWB impulse radio applications.