The authors have analyzed and measured the dispersion effects of residual chirp arising from asymmetry in field overlap in Mach-Zehnder structures. A clear influence of chirp was seen at 7 Gb/s and a propagation length of 75-km nondispersion shifted fiber at lambda =1.55 mu m. It is shown that a MZ-modulator with field overlap in one arm only and operated in the blue-shift mode is the best choice for single coplanar stripline (CPS) electrode MZ-modulators in this case. Calculations have shown good agreement with measured results. This indicates that a simple model for fiber mode propagation have shown good agreement with measured results. This can be used together with a FFT-algorithm for calculating the effect of residual chirp from external modulators in multigigabit dispersive fiber-optic systems.