Design and Implement of QPSK Modem Based on FPGA. Abstract : This paper presents a method to designs QPSK modulator and demodulator of a spread spectrum system which use field programmable device. The method uses the tool of Quartus Ⅱ of American Altera Co. The whole system is divided into several small models based on top-down design method, and using VHDL hardware description language to design each model.