| The atomic clock uses the precise and stable quantum transition of the quantum system to obtain the standard frequency,and the output time-frequency signal is the physical quantity with the highest measurement accuracy so far.Since the birth of atomic clocks,its development direction has been continuously expanded,and the research on miniaturization and miniaturization has attracted much attention.At present,miniaturized atomic clocks are mainly based on passive coherent population trapping（CPT）,which can realize miniaturization and miniaturization of atomic clocks because of the elimination of microwave cavity.The current chip atomic clock is based on CPT,only the size of a matchbox,the power consumption is less than 120 m W,the weight is less than 35 grams,and its frequency stability is at the level of 10-10τ-1/2（τ≤1000s）.And it still has a lot of room for improvement to meet applications with limited volume,power,weight and higher time stability requirements,such as the next-generation Beidou navigation system,low-orbit high-speed communication,and supersonic weapon guidance.At present,the high-performance atomic clock based on CPT has been preliminary studied,represented by the Paris Observatory in Paris and the Femto-ST laboratory in Besan?on,and its frequency stability has reached 1-3×10-13τ-1/2（τ≤100s）level.In order to realize the high-performance CPT atomic clock(1×10-13@1s)in China and further explore its frequency stability limit,higher requirements are put forward for the microwave used to excite the transition spectrum of the atomic ground state clock.According to the intermodulation effect and the contribution of microwave power to the frequency stability of atomic clocks requires microwave phase noise and microwave power stability to reach≤-109 d Bc/Hz and≤1.5×10-5 W,respectively.In this context,the study of low-noise microwave frequency synthesizers has become an important research content of CPT atomic clocks.This paper studies the frequency synthesizers mainly used in CPT atomic clocks.Firstly,the research and development status of CPT atomic clock and frequency synthesizer is introduced and the relationship between them is discussed.After that,the domestic and foreign research background of frequency synthesis technology and various frequency synthesis technologies are introduced,and the advantages and disadvantages of current related technical means are drawn.Therefore,a design scheme of microwave frequency synthesizer based on the nearest frequency multiplication factor is proposed,and a microwave frequency synthesizer with an output frequency of 3.417 GHz is built and realized.The synthesizer is mainly composed of an oven-controlled crystal oscillator,a surface acoustic wave filter,a direct digital frequency synthesizer and a phase-locked dielectric oscillator.It can flexibly change the frequency multiplication factor and small frequency value in the design structure and then adjust the output frequency to adapt to different atomic clocks.After that,the absolute phase noise,residual phase noise and microwave power stability of the key signals in the microwave frequency synthesizer were measured,and the phase noise limiting factors and the space for optimization in the design scheme were analyzed through the measurement results.Then,the coherent dichroic phase noise generated by the microwave coupling into the laser was measured,and the influence of the intermodulation effect on its short-term frequency stability was estimated.Finally,this paper compares the relative intensity noise（RIN）before and after the laser with and without microwave modulation,and further analyzes the influence on its frequency stability.From the above experimental test results,it can be seen that the absolute phase noise of the output 3.417 GHz signal at the frequency offsets of 1 Hz,10 Hz,100 Hz,1 k Hz and 10 k Hz is-53.0 d Bc/Hz,-83.3 d Bc/Hz,-107.7 d Bc/Hz,-119.2 d Bc/Hz and-124.0 d Bc/Hz.The contribution of the synthesizer phase noise to the frequency stability of the CPT atomic clock through the intermodulation effect is 5.81×10-14 at 1 s.Further considering the possible phase noise deterioration when microwaves are loaded on the laser to generate coherent dichromatic light,by measuring the phase noise of the optical beat signal at 6.834 GHz,its contribution to the frequency stability of the CPT atomic clock is 5.95×10-14 at 1 s.When the microwave power is 26.9 d Bm,its relative power stability is 2.78×10-5,and its contribution to the frequency stability of the CPT atomic clock is 8.64×10-14 in 1 s average time.Compared to the synthesizer scheme before this experiment,the frequency stability contribution is reduced by 2.4 times.This synthesizer scheme can meet the requirements of low phase noise and high frequency stability,and is a feasible scheme for designing low phase noise frequency synthesizers for atomic clocks. |
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