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A SIS mixer with RF bandwidth of 1 octave is designed, fabricated, and characterized. The mixer chip consists of 8 Nb/AlOx/Nb junctions connected in parallel through Nb/SiO2/Nb microstrip lines (MSL), a twin-slot antenna, and a 3-stage MSL transformer. Experimentally, the 2.5 dB RF band is found to be 230-440 GHz, which agrees quantitatively with theoretical calculation. Receiver noise temperature Trx is measured as 260-680 K in the band. After correction for IF-amplifler noise and beam-splitter reflectivity, the noise is 170-320 K, i.e., 13-20 times of the quantum noise. The sensitivity flatness in the band can be improved to 1.9 dB by further optimization of the transformer.