The error of photoelectric rotary encoder mainly comes from interpolation. The quartering deviation of Moiré fringe of a rotary encoder is the most important part in interpolation error. The two methods of precise phase correcting are designed. Those correcting circuits are simple and efficient. The sine signal of encoder is regarded as phase reference(0°)

the cosine signal can be separated into 0°and 90° vectors .The quartering deviation is created by 0° vector mainly. In this paper

the cosine signal is quartered to sine signal through compensating the 0° vector of cosine to reduce the quartering deviation. If the lower error is needed

the two signals

sine signal and inverting signal of sine

and cosine signal can be created

then the two signals can become quartering seriously though adjusting the value of cosine equal to sine signal. The experimental results show that the interpolation error is reduced from 1″ to 0.1″ after the precise phase correction.