A capacitive rotary encoder for angular position and angular speed measurements is designed. The transmitting segments applies four square wave voltages with phase differences of 90° to provide a modulated electric field. The patterned movable plate based on sinusoidal function can quadrature modulate angular position encoding to phase/frequency modulated signals. The mechanical angle is modulated to phase position of sine and cosine signals in a field programmable gate array processor based on the quadrature demodulation and the coordinate rotational digital computer algorithm.