Spurious Research And Design For The Dds Signal Generator

The invention of Direct Digital Synthesis (DDS) is the revolution of signal frequency synthesis technology. DDS is a wholly digital structure circuit with variousadvantages such as fast frequency switching speed, high frequency resolution, lowphase noise and maintaining constant phase while frequency hopping. In the recent decade, it has been widely used in the fields of radar, communication, apparatus, etc. For the limitation of digital hardware operating speed, Nyquist theorem and DDS structure, the spurious performance of DDS' output is bad and the output frequency can' t reach a high level.In this dissertation, a systematic research is made on DDS. The development of the frequency synthesis is introduced. The output spectrum of the DDS on the presence of both phase and amplitude truncation is analyzed by computer simulation. An analysis is also made on the effect of the DAC's nonlinearity. Based on analyzing the output spectrum of the DDS, some methods to reduce spurs are proposed. They are modified random dithering and Rom compressing techniques. Then a sine/cosine generator is researched. In this structure, the spurious performance is no longer restricted by the capacity of the ROM. A seven-order ellipse Filter Circuit which reduce the noise of the signal above 20MHz is designed to get little spurs, and desin a new process method to lower the DDS' output spurs which resolve the the problem that process eleven character with the four character variable.Finally, combined with the practical engineering, the controlling circuit of the low spurs and wideband DDS signal.All in a word, DDS is one of the most advanced frequency synthesizing techniques, but there are still some disadvantages in using DDS, for example, the narrow bandwidth, which more or less limits its application. In the last part of the thesis, some suggestion are proposed to further improve the result of frequency synthesis.