Abstract: Spreading band with negative feedback is a valid method that is presently widely used at home and abroad. From developing the modulation summator of microwave phase modulator, it is found that the modulation summator presented in this paper seems to bp an another valid method of spreading band. The gain-band product of the latter is much larger than that of the former.The new modulation summator preseuted in this paper has a much wider band, higher non-distorted output voltage aud better rejection capability to combined-wave than Goddard tracking system developed by Motorola Co. Its characteristics are analysed in detail with the flow-graph. A series of formulas are derived. It is found that the theoretical analysis agrees well with experimental curve. Its circuits are very simple and work stably within the temperature of 40+85℃.