Design And Implementation Of Low Phase Noise Frequency Synthesizer Based On DDS

Firstly,this paper introduces the background and development status of frequency synthesizer.With the development of communication technology and measuring instrument technology,the requirements for frequency synthesizers are increasing.This paper has designed the scheme of the frequency synthesizer based on the requirement of system.The general low phase noise system is designed based on the phase-locked loop,but the phase-locked loop has the disadvantages of slow frequency switching and low frequency resolution.The DDS has the advantages of high frequency conversion and high frequency resolution.This paper aims to combine the advantages of both the PLL and DDS.By comparing several structures of frequency synthesizer,the PLL incited DDS structure is finally determined as the final scheme for design.This paper is mainly divided into two parts of low phase noise PLL design and suppression of DDS spurious.This paper analyzes the phase noise of phase-locked loops with different structures and the effect of loop filter on phase noise.It finds out that the phase noise suppression effect of the loop filter on the oscillator in the transition band is not high.For systems with high phase noise requirement,the oscillator's noise will affect the output.Based on the parallel correction structure,a nested phase-locked loop structure is designed.This structure can enhance the loop filter's suppression of in-band phase noise.Finally,through the implementation of the hardware circuit,a reference signal source with phase noise of-113.22dBc/Hz@1kHz,and-125.91dBc/Hz@10kHz at 6GHz is developed.This paper analyzes the principle of DDS spurious generation.Combining the compression look-up table and Sunderland's spur suppression method,this paper improves the existing method and proposes a new phase truncation spur suppression method.The DDS model is built on the MATLAB platform and the simulation of the new algorithm is performed.The new algorithm can meet the 70 dBc system requirements for DDS spur suppression.