^85Rb喷泉钟的低相位噪声微波频率综合器设计

Design of Low Phase Noise Microwave Frequency Synthesizer for ^85Rb Fountain Clock

采用低相位噪声频率合成和锁相技术,为世界首台^85Rb喷泉钟研制了低相位噪声微波频率综合器,实现了低相位噪声的钟频信号输出。当该微波频率综合器的频率分辨率为0.7μHz,输出频率为3.036 GHz时,在傅里叶频率1 Hz,100 Hz和10 kHz处的单边带相位噪声谱密度分别为-97 dBc·Hz^-1,-127 dBc·Hz^-1和-130 dBc·Hz^-1,剩余相位噪声比本机振荡器绝对噪声低20 dB以上。研究了量子散弹噪声和Dick效应,发现在微波链路噪声为主要噪声的情况下,^85Rb喷泉钟的稳定度与目前^87Rb和^133Cs喷泉钟相当。微波系统对^85Rb喷泉钟稳定度的贡献为2.9×10-^13τ-1/2,其中频率综合器的剩余相位噪声贡献为1.2×10-^14τ-1/2(τ为积分时间)。利用该微波综合器,可以实现^85Rb喷泉钟的高稳定运行,并为其性能的提升打下基础。

We have developed a low-noise microwave frequency synthesizer for the world's first ^85Rb fountain clock.The setup is based on the techniques of low phase-noise frequency synthesis and phase-locking,which can realize the clock frequency signal output with low phase noise.When the output frequency is about 3.035 GHz and the frequency resolution is 0.7μHz,the single-sideband phase noise densities at 1 Hz,100 Hz,and 10 kHz Fourier frequency are-97 dBc·Hz^-1,-127 dBc·Hz^-1,and-130 dBc·Hz^-1,respectively.Moreover,the residual phase noise of the microwave frequency synthesizer is lower by 20 dB than that of the local oscillator.By comparing the contributions of quantum project noise and Dick effect,we show that the stability of the ^85Rb fountain clock is comparable to those of ^87Rb clock and ^133Cs clock,where the noise from a microwave system is the main noise source.The stability caused by the microwave source for the ^85Rb fountain clock is estimated as 2.9×10^-13τ-1/2(τis integration time),among which the contribution of the residual phase noise from the frequency synthesizer is 1.2×10^-14τ-1/2.The synthesizer is helpful to realize the operation of the ^85Rb fountain clock with high stability and is expected to be the basis of the future improvement in clock performances.