Abstract: Direct Digital Synthesis is a new kind of frequency synthesizing technology that is widely applied in arbitrary waveform generator. This paper introduces a realization of DDS, which is based on programmable device. It also presented a method of realizing frequency modulation by real-time changing frequency tuning word of high-speed phase accumulator, Frequency sweep, FM and FSK modulation are analyzed and designed. The data can be built stably and the frequency be synthesized is correct.