Research And Design Of Spurious Reduction Of Direct Digital Frequency Synthesizer

With the rapid development of modern communication system and digital integrated circuits,high quality signal sources are becoming more and more important in the fields of digital communication,space communication,radar measurements,electronic warfare and satellite navigation.Compared with the Direct Synthesizer and PLL Synthesizer,DDS has been widely used in modern digital communication system with the advantages of high frequency resolution,fast frequency switching,continuous phase and synthesis complex waveform easily.As the main development direction of frequency synthesis technology,DDS has many advantages.However,its spurious performance is far below the system requirements,which has seriously hampered the development of DDS technology.In this paper,the DDS system with good flexibility and spurious reduction performance is studied and designed.In order to improve the spurious reduction characteristics of DDS,the spurious sources,spurious spectral characteristics and existing spurious reduction algorithms of DDS are analyzed and studied.In view of the influence of phase truncation error on the spectral purity of DDS digital part,an new interpolation-based algorithm of none phase truncation is proposed.Then,based on the analysis of the proposed algorithm by Matlab,the interpolation-based algorithm of none phase truncation can effectively reduce the hardware overhead and improve the DDS spurious reduction performance by improving the ROM structure and lookup table mechanism.In view of the primary limitation of DDS performance of complex analog device DAC,an all-digital DDS architecture based on Delta-Sigma digital modulation technology is implemented.A 1-bit4-order low-pass Delta-Sigma digital modulator and a 1-bit 8-order band-pass Delta-Sigma digital modulator are designed and the Simulink platform is used to model and simulate the Delta-Sigma digital modulator.The simulation results show that the SNR can reach100.7dB,ENOB is 16.In this paper,an optimized DDS with low spurious is designed and implemented,as well as the key phase-to-amplitude converter and Delta-Sigma digital modulator.The optimized DDS can achieve 32-bit frequency control accuracy and 14-bit phase control accuracy,implement complex digital modulation such as FM,PM,2FSK and 2PSK,and have two modes of linear sweep modulation.In this paper,the function verification of the phase-to-amplitude converter,Delta-Sigma digital modulator and DDS circuit are carried out respectively in Matlab and Modelsim environments.The verification results show that the phase-to-amplitude converter functions properly,the SFDR can reach-86.9d Bc;the Delta-Sigma digital modulator functions properly,the SNR can reach 99.4dB,ENOB is 16;the frequency accuracy in magnitude between 10^-9.In addition,the circuit performance evaluation of the phase-amplitude converter and Delta-Sigma digital modulator are carried out based on FPGA,and the DDS circuit was synthesized with the Design Compiler software with the SMIC 0.25 standard cell library.