Direct Digital Frequency Synthesis Technique In Software Radio

Direct Digital Frequency Synthesis (DDS) is a new frequency synthesizing technique which is developed rapidly in recent years. DDS has many advantages such as higher frequency resolution, short frequency changing time, and continuous-phase in frequency changing , etc. Since DDS is programmable and has a digital architecture, DDS is used widely and plays a very important part in Software Radio system. But the serious spurs in the output spectrum of DDS limits the further usage of DDS.In this paper, we have a whole analysis on the spectrum performance of DDS. The derivation on spurs performance was made, which is mainly decided by the phase truncation, amplitude quantizing and glitches of DAC. And the results of simulation testified the conclusions. Based on the analysis, we bring up a new way of thinking, after considering the characteristics of the DDS design in Software Radio system platform. We adopt the ROM compression technique of DDS special chip and other optimizing methods in the design. We still bring up a new modified method, of which the compression ratio is 139.6:1. These methods are effective, and the spur performance of DDS was improving greatly. In order to simplify the output spectrum analysis of DDS, we build a DDS simulation platform. The results of simulation proved that the optimizing methods we used are very useful, and an improvement about 30 dB was got in the DDS digital output spectrum.