Research On Accurate Frequency Synthesis Of DDS Based On FPGA

Frequency synthesis,also known as frequency synthesis,refers to the process of synthesizing discrete frequencies with the same index from a reference source with better frequency stability and accuracy in the frequency domain.The circuit system that realizes frequency synthesis is called frequency synthesizer.Frequency synthesizer is an important part of modern electronic system or equipment.It can be used as an excitation signal source of transmitter or a local oscillator of receiver in aerospace and communication equipment.It can be used as an interference signal generator in electronic countermeasures equipment and as a standard signal source in test equipment.Frequency synthesis technology is mainly divided into three categories: direct analog frequency synthesis,phase-locked frequency synthesis and direct digital synthesizer(DDS).Because it has the advantages of easy integration,high frequency hopping speed,high frequency resolution,and continuous phase when switching frequency,DDS has become the preferred scheme for designing and realizing frequency synthesizer.However,with the development of science and technology and the progress of technology,modern electronic systems or equipment have higher and higher requirements for the output signal of frequency synthesizer,and the drawbacks of traditional DDS frequency synthesizer methods have gradually emerged.For example,some special frequency points can not be accurately synthesized and the output frequency spurious is relatively large.This paper improves the traditional DDS frequency synthesis method and develops a frequency synthesizer based on Field Programmable Gate Array(FPGA).The main work is as follows:(1)A DDS frequency synthesizer based on FPGA is designed and implemented,which provides a test platform for verifying the improved DDS method proposed in this paper.(2)Aiming at the frequency error caused by phase truncation of traditional DDS,an accurate DDS method using compound frequency control words is proposed.This method adjusts the accumulation mode of the phase accumulator,which can increase the controllability of the output frequency signal and improve the accuracy of the signal.(3)A piecewise linear approximation method is proposed to suppress the spurious noise caused by ROM amplitude quantization in traditional DDS.By calculating the missing part of the magnitude information through the limited magnitude in ROM,this method can improve the magnitude quantization degree in ROM and effectively suppress spurious.The simulation and experimental results fully prove the effectiveness of the improved method proposed in this paper.Compared with the traditional DDS synthesis method,the new method improves the accuracy by three orders of magnitude(the relative frequency deviation of synthetic frequency is about 2.0E-12)and suppresses the harmonic spurious by 26 dBm on the basis of ensuring the stability of synthetic signal.