The thesis proposes a third order continuous-time sigma delta modulator used in GSM. We used a special 1.5bit quantizer, and to use its three different states to reach a differential feedback path. That can improve the resolution of our circuit. Oversampling and noise shaping are two keys of sigma delta modulator. In structure, the continuous-time features can reduce power consumption. The proposed sigma delta modulator uses TSMC 0.35 m CMOS process and its sampling frequency is 10.8MHz, bandwidth is200KHz and oversampling ratio is 32.