Study On Frequency Synthesizer With Ultra-low Phase Noise

At present,frequency synthesis technology is more and more widely used in modern integrated circuit design,and it is also an important part of modern communication technology.Because in any wireless communications,optical fiber communications,highperformance digital circuits are involved in the frequency synthesizer.Therefore,the design of a high performance frequency synthesizer is particularly important.It is so popular because of its features of narrowband,tracking and filtering.With the progress of science and technology,radar technology has been deeply studied and widely used in various fields,and phase noise is an important factor affecting the performance of radar system.Therefore,when we study the frequency synthesizer,the study of its phase noise is more and more demanding.Based on this requirement,a frequency synthesizer with ultra-low phase noise is designed in this theise.The traditional frequency synthesizer is a self-regulating negative feedback system,which is mainly composed of phase discriminator,loop filter,voltage controlled oscillator and frequency divider.All these four parts contribute to the phase noise of the system,so how to improve the total phase noise is closely related to the performance of each part.In this thesis,the phase noise contribution of these four parts is systematically analyzed,and it is concluded that the components with the greatest influence on the loop noise are VCO and frequency division ratio N.So according to this theory in this thesis,a new scheme to solve the problem of high phase noise,through the analysis and comparison of several schemes,and finally confirmed by mixing scheme,which applied the comb spectrum generator to complete the function of frequency doubling,by step in the comb spectrum generator diode nonlinear characteristics,have used together with filter,can very good reduce additional phase noise,which can well improve the total phase noise of system.In this way,higher requirements for phase noise can be satisfied.This thesis will study from the following aspects:1.Analyze the phase noise contribution of each component of the frequency synthesizer,and conclude that the phase noise of each component is obtained by the product of the inherent noise and the transmission function in the system.2.By analyzing the phase noise of the single-ring frequency synthesizer,it is found that the frequency division ratio N of the divider in the loop will worsen the phase noise of the system,which will have the added value of 20 log N.3.In order to reduce the phase noise of frequency division ratio N,a dual-loop scheme,a frequency-doubling scheme and a frequency-mixing scheme are proposed respectively.These schemes are analyzed and compared respectively,and finally a frequency-mixing scheme is determined to reduce the phase noise value of the system.4.Design the comb spectrum generator,make its output meet the requirements of the scheme,reprocess the comb spectrum generator,and test the object with matching filters.5.ADF4371 chip was selected as a single-ring frequency synthesizer,and its circuit diagram and PCB board were drawn,and the real object was processed,and the test results were used as a reference for comparison.6.Select the appropriate chip to complete the circuit design of the mixed-frequency synthesizer,process the real object,and then test the real object.The final result is compared with the phase noise of the traditional frequency synthesizer to illustrate the feasibility of this scheme.