Research On Spurious Reduction Techniques Of DDS

The Direct Digital Frequency Synthesis(DDS) is a kind of new frequeney synthesis method and also the third revolution in the frequency synthesis techniques. The direct digital frequency synthesizer consists of the the phase accumulator,the sine look-up table,the digital to analog ceonverter and the low band filter. It is of high frequeney resolution, fast frequeney switching speed,low phase noise.the ability to switch frequeneies while maintaining constant phase,and the ability to producing arbitrary waveforms.Because of these features,DDS can be widely used in such fields as radar,communications,electronic warefar an electonic measurement instruments.But DDS is also of low output frequeney, spurious performance unsatisfactory,which limit the development of DDS.At first, in this dissertation, the development of frequency synthesis is introduced, then the direct digital frequency technique and its spurious are mainly studied, the spectrum characteristic of ideal direct digital frequency and spurious sources are analyzed. Then the spectrm charaeteristies of the phase truncation error and amplitude quantization error are analyzed by the method of Fourier series.The method of spurious reduction is sdudied,including ROM compression, random phase jitter and phase superposition.and the last two methods are simulated by computer.In addtion,the article discussed the frequeney synthesis of DDS+PLL, which is modular simulated in Simulink.Comparing with the phase superposition method the effect of reducing DDS spurious which comes from phase jitter is more obvious.Because of the right phase dither injection to the DDS disrupt the period of phase truncation error sequence,The error power spectrum is changed from discrete uniform distribution line into broadband noise,Although the DDS is of small output step size,low phase noise,there are more disadvantanges spurious.Otherwise,PLL is of the high phase noise when output step is small, but it was of good spurious suppression performance. Therefore, the DDS+PLL of frequency synthesizer which are combined effective is proposed as a more reasonable frequency synthesizer program. enhancing the signal of a noise power spectral density ratio.