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We investigate the performance limits and the impact on the system margin of 10-Gb/s duobinary modulations due to reducing the bandwidths of single-arm Mach-Zehnder (MZ) modulators in a 10-Gb/s duobinary transmitter through numerical simulation. In addition to that, the individual and combined effects of dc-bias offset ratio to switching voltage and residual chirp due to the asymmetry of modulators on the system performance have been studied using reduced bandwidth single-arm MZ modulators. Our results suggest that with a single-arm MZ modulator of bandwidth reduced to 5 GHz and a duobinary filter of 2.6-GHz bandwidth, the sensitivity penalty can be maintained within 2 dB over a 200-km span of uncompensated single-mode fiber referred to that at 0 km of ideal duobinary transmitters. In this case, the allowable range of the residual-chirp parameter is to be from -0.1 to +0.1 with dc-bias offset ratio of less than /spl plusmn/2.5%.