therefore, when the phase drift produced by the ladder wave in the reset period is ∅_m-π, the output of interference signal in reset period have opposite sign compare with other, which makes the closed-loop control balanced while the scale factor doubled. Generally, the conditions of components are random at the moment of starting. It is impossible that the phase drift of the system is 2π .To guarantee the stability of work point of the digital closed-loop control, we must constrain the amplitude of ladder wave, which makes the changes of the phase drift limited between -2π and 2π . In the consideration that the changes of phase drift produced by the drift of the Y waveguides modulate degree is not so obvious, the limit of compensation should be smaller to guarantee the stability of the work point of closed-loop control.