Research And Design Of Low Noise RF Frequency Synthesizer For Optical Clock System

With the growing scientific and technological progress and development,people are striving for high precision measurement.As frequency is the highest precision physical quantity in modern physics,the testing of many different physical quantities is converted to the testing of frequency,which needs to seek a more accurate frequency standard.The optical clock can enhance the accuracy of frequency standards to E-18 or better,but it only outputs laser light at a specific frequency.If we want to carry out high-precision measurements with an optical clock,we must transfer the frequency stability,coherence and accuracy of the optical clock to other optical or microwave bands.Thus,a frequency synthesizer with superior phase noise is the best instrument to achieve this goal.In accordance with the demands of the optical clock system,this paper designs a low-noise RF frequency synthesizer for optical clock systems with an output frequency range of 50 MHz to 500 MHz and a frequency resolution of 1 ?Hz.The test results show that the phase noise performance is-88.39 d Bc/Hz@10Hz,-116.19 d Bc/Hz@100Hz,-128.09 d Bc/Hz@1k Hz,-131.76 d Bc/Hz@10k Hz,-131.03 d Bc/Hz@100k Hz.The results obtained during the theoretical study and circuit design of this frequency synthesizer are as follows.1.By researching and analyzing the performance requirements of the frequency synthesizers for optical clock system,we give the current studies on low-noise frequency synthesizers by domestic and foreign scholars in recent years,summarize the current common structure of low-noise frequency synthesizers and the dominant performance metrics,and determine the phase noise and other performance metrics of the proposed low-noise RF frequency synthesizers.2.The theory associated with frequency synthesizers is studied.Firstly,the concept of phase noise is introduced.Meanwhile the composition of phase noise in frequency synthesizer and its sources are analyzed.Secondly,the transformation of the phase noise during frequency synthesis is derived using mathematical equations,and the equivalent phase noise model of the frequency synthesis system is proposed.In this model,the phase noise is mainly contributed by two ways.The first part comes from the input signal.The phase noise that it transfers to the output follows a strict mathematical formula,and the calculation matches the actual result.The second part comes from the devices in the synthesizer and includes white phase noise,flicker phase noise and environment-originated phase noise.The sum of the white noise of all the devices in the synthesis system constitutes the white phase noise,which is expressed in the phase noise curve as the noise floor of the signal.Other phase noise can be eliminated or optimized during the synthesis process based on their correlation.Finally,the frequency stability of the frequency synthesizer is analyzed and the derivation process of the Allan's variance is given.3.The system scheme of this low-noise RF frequency synthesizer is proposed,which uses hybrid method for frequency synthesis,and its main modules include: comb spectrum generator,point frequency synthesis module,DDS,PLL,bandwidth extension module and target frequency synthesis module.The comb spectrum generator and the point frequency synthesis module generate low noise frequency points,and then these frequency points are mixed with the output signal of DDS to expand the 1MHz bandwidth into a 10 MHz bandwidth.Finally,the target frequency synthesis module is used to combine the 10 MHz bandwidth reference frequency with the PLL's output signal and the signal generated by the comb spectrum generator to produce the terminal output frequency and its range is 50 MHz to 500 MHz.The main performance indexes of the system are also analyzed theoretically.The phase noise can reach-136 d Bc/Hz@10k Hz and the frequency resolution can reach 0.035?Hz.4.The operating principle of each module of the proposed frequency synthesizer is specifically introduced.Then the module circuit design is given.Based on the selected device parameters as well as the proposed model,the simulation is performed by ADS to verify the function of each module circuit.5.The PCB drawing of the frequency synthesizer was completed using Altium Designer tool,and the test work was carried out for each module as well as the overall performance.The test results of each module are as follows: the output spectrum of the comb spectrum generator has a good flatness,and the output power is all around-16 d Bm.The test results match the simulation results.The output phase noise of the point frequency synthesis module,PLL and DDS all meet the system design requirements,and leave a certain performance margin.After completing the performance testing of the module circuits,the connection between each modele is made and the phase noise test of the system is conducted.The phase noise of the system is-88.39 d Bc/Hz@10Hz,-116.19 d Bc/Hz@100Hz,-128.09 d Bc/Hz@1k Hz,-131.76 d Bc/Hz@10k Hz,-131.03 d Bc/Hz@100k Hz.The experimental results of system satisfy the design requirements.