Study On Spurious Reduction Techniques Of DDS

The Direct Digital Frequency Synthesis (DDS) is a new frequency synthesis method and also a revolution in the frequency synthesis techniques. The direct digital frequency synthesizer is a fully digitalized frequency synthesizer, which consists of a phase accumulator, a sine look-up table, a D/A converter and a low band filter. It is of high frequency resolution, fast frequency switching rate, low phase noise, the ability to switch frequencies while maintaining constant phase, and the ability to producing arbitrary waveforms. Because of these features, DDS can be used in such broad fields as radar, communications, electronic warfare and electronic measurement instruments.The software radio technology was also a hot spot in wireless communications in recent years. It is in the direction where the technology will go. And the signal digitization is its key step. Since DDS is programmable and has a digital architecture, it is used widely and plays a very important part in Software Radio system. But the spurious performance of DDS'output is unsatisfactory and the output frequency limits its further utilities.This thesis introduced the development of frequency synthesis techniques and common frequency synthesizer techniques .The direct digital frequency technique and its spurs are mainly studied. At first, analyzes the spectrum characteristic of ideal direct digital frequency synthesizer and spurs sources. And then the output spectrum of the DDS on the presence of both phase and amplitude truncation is analyzed. Also, an analysis is made on the effect of the DAC's nonlinearity. Based on the analysis of the output spectrum of the DDS, some methods to reduce spurs are proposed. They are Rom compressing techniques(include using the sine function symmetry and decomposes the ROM table reading for reads over cursorily and reads attentively), uses the frequency control word and 2 B is the prime number and random dithering techniques(include frequency dithering, phase dithering and amplitude dithering) , use random dithering techniques to improve spectrum are mainly studied. Based on phase dithering technique, a new structure of DDS to improve spectrum which caused by phase truncation, Finally the computer simulation is fulfilled.In the phase to amplitude converter of DDS, another task this thesis did is the introduction of two conversion methods: look-up table method and calculation method, of which CORDIC algorithm is mainly studied. Furthermore, a sine/cosine generator based on CORDIC algorithm calculate sine value and cosine value is researched. With ROM economy, the CORDIC algorithm was adopted to produce sine and cosine values. It works faster with simple shifting and adding operations. Thus make the FPGA implementation feasible. Moreover, CORDIC algorithm achieved spurious performance successfully circumvented the ROM capacity's limitation.