Abstract: This article analyses the improvement to S/N of Sigma-delta. It introduces the application of sigma-delta in A/D for digit fractional divider, and describes the improvement to output phase vibration in fractional divider due to the application of sigma-delta. The conception of replacing multiple accumulators with repeatedly cycled utilization of a sigma-delta accumulator is raised.

Key words: sigma-delta, fractional divider, accumulator, convertor