Research On Stability Of Hybrid Frequency Synthesizer With In-loop Mixing

At this stage,the traditional single frequency synthesis technology is no longer enough to meet the demand.The hybrid frequency synthesizer composed of multiple frequency synthesis technologies combined with each other has become a research hotspot and trend,but hybrid frequency synthesizer whose complex system structure with loop instability has become a difficult point in practical applications.The purpose of this paper is to study the stability of hybrid frequency synthesizers.Choose the hybrid frequency synthesizer of PLL and DDS with in-loop mixing as the research object,It not only uses theoretical analysis methods to mathematically model the system to analyze its stability,but also uses ADS software builds a system simulation model for verification,and studies the effects of DDS,CP-PFD,Mixer,and BPF components in the hybrid frequency synthesizer on the stability of system.Firstly,the basic principles of the hybrid frequency synthesizer with in-loop mixing including PLL,DDS,Mixer and BPF are introduced in detail.Not only used ADS software to build PLL's phase noise model and time domain simulation model for simulation verification and performance analysis of PLL,but also used MATLAB Simulink simulation tools to model DDS to obtain an ideal simulation model and verify the theoretical analysis.Secondly,in view of the different working conditions of the hybrid frequency synthesizer with in-loop mixing,the linear and nonlinear stability theory analysis methods are used to mathematically model the system.Among them,the linear analysis method adopts classical analysis system stability methods such as Bode diagram,Root Locus method,and Routh–Hurwitz criterion.Based on the open and closed loop transfer function of the system,the parameters of unlocked system and locked system are substituted for verification.The stable system can be directly judged by observing the distribution position of the poles and zeros of the system and the related calculation results.Compared with the linear analysis method,the nonlinear analysis method is more accurate to judge the stability of the system.It is a behavior-driven modeling for the nonlinear characteristics of CP-PFD,and the control voltage and phase difference of the frequency synthesizer are predicted by calculation at the next working time.It can judge the working status of the frequency synthesizer at any moment,so as to judge the stability of the system.By building two different frequency synthesizer system models with in-loop mixing structures for comparison,a frequency synthesizer with in-loop mixing structure that mixes before frequency division with ideal stability is selected so that the loop parameters can be changed subsequently to stabilize the system analysis.In view of the typical phenomena that affect the stability of the system in the system simulation process,such as the PLL cycle-slip and the non-linear phenomenon of Mixer,combined with the simulation,it is found that the PLL cycle-slip phenomenon is related to the small capacitance parameter of the LPF.The non-linear phenomenon of Mixer is related to the performance design of the BPF.By optimizing the capacitance parameters of the LPF and the performance parameters of the BPF,these phenomena that cause system instability can be improved or even eliminated.Finally,the factors that affect the stability of the hybrid frequency synthesizer with in-loop mixing are studied,including the influence of phase noise,non-linear devices such as CP-PFD and Mixer,DDS output frequency range,and spurs on the loop stability.Through theoretical analysis,combined with the simulation results of building a frequency synthesizer simulation model,the effectiveness of the stability theory analysis method is verified.