DDS-Based PLL Frequency Synthesis Technology

Frequency synthesis technology is to use high-stability and high-precision reference information sources to generate signals of different frequency bands or different modes through certain technologies.Frequency synthesizer is the core component of many electronic systems,and its performance indicators are related to the performance of the entire electronic system,such as digital communication bit error rate,satellite positioning accuracy,radar identification,communication anti-jamming performance and so on.Therefore,how to make the frequency synthesizer more integrated and have better performance indicators has become the main direction of current research.This paper firstly introduces the theoretical basis of direct digital frequency synthesizer(DDS)and phase-locked loop(PLL),including the application of the two in radar,radio frequency and other fields and their respective advantages and disadvantages,and then combines the above two technologies to design two The frequency sources with different emphases are as follows:(1)The first frequency source is an FPGA-based DDS signal source,which realizes functions such as linear frequency modulation,nonlinear frequency hopping,m-sequence pseudo-random frequency hopping,pulse modulation,and modulation.This design completed the PCB layout design,processed and built the frequency source platform,and designed a host computer for signal processing and data interaction according to related functions.Among them,the DDS module adopts the AD9914 chip,and its frequency resolution can reach the order of p Hz.The FPGA module is mainly used to communicate with the host computer and read and write DDS registers.The main innovation of this design lies in the algorithm design of nonlinear frequency hopping,which avoids the problem of excessive consumption of FPGA resources in traditional nonlinear frequency modulation,and can realize frequency modulation of arbitrary modulation signals,including sinusoidal frequency modulation,tangential frequency modulation and so on.Signal processing and frequency control words are calculated by the host computer,while FPGA is only responsible for basic logic control,data storage and DDS data reading and writing,and the resource consumption is very small.In addition,this design uses the short-time Fourier transform(STFT)to analyze the output signal to ensure the accuracy of the nonlinear frequency hopping signal.The main innovation of this design is the algorithm design of nonlinear frequency hopping,which avoids the problem that traditional nonlinear frequency modulation consumes too much FPGA resources,and can realize the frequency modulation of arbitrary modulated signals,including sine frequency modulation,tangent frequency modulation,etc.The signal processing and frequency control word are calculated by the upper computer,while the FPGA is only responsible for basic logic control,data storage and DDS data reading and writing,and the resource consumption is very small.In addition,this design uses short-time Fourier transform(STFT)to analyze the output signal to ensure the accuracy of nonlinear frequency hopping signal.(2)The second frequency source is a high-bandwidth low-spurious small-step frequency source based on DDS and PLL.The DDS module in the above-mentioned signal source is used as the reference source of the phase-locked loop LMX2594.After the two are cascaded Increase the output bandwidth of DDS through the frequency multiplication of the phase-locked loop,and finally realize the output bandwidth of7.5G～15GHz.The main innovation of this design lies in the design of low spurs.The frequency synthesis scheme of traditional DDS excitation PLL often has insufficient spurious suppression.This design will avoid the spurs caused by fractional frequency division,and only use integer frequency division to achieve small steps.At the same time,the spurious suppression degree is improved.After the frequency sweep test,the final spurious suppression degree is around 65 d Bc,and the frequency step is within 100 Hz.