Active Compensation Scheme For Low Frequency Stable Transmission Based On PLL

Stable and accurate frequency synchronization technology is critical in many applications,including both the industry and military needs,such as the square kilometer array,comparison of atomic clocks,and global navigation satellite system.With the development of advanced high-precision frequency standard sources,frequency synchronization technology has begun to face many challenges.How to stably transmit ultra-high precision standard frequencies from local to remote sites is a problem that needs to be solved worldwide.The traditional frequency transmission system is based on the satellites.However,due to multipath effects during transmission,tropospheric and ionospheric delays,the signal-to-noise ratio of satellite signals will deteriorate seriously,resulting in poor stability of the frequency signal.So it not enough to meet the needs of high precision applications.In contrast,optical fiber is considered to be an excellent transmission medium for replacing traditional satellite links due to its advantages of anti-electromagnetic interference and low loss.However,mechanical stress and temperature changes may cause changes in the transmission characteristics of long-distance fibers,and introducing random phase jitter to reduce the stability of the transmission frequency.In order to improve the stability of the final signal at the far end,it is necessary to measure and compensate for the phase change introduced by the fiber link.A variety of fiber-based stable frequency transmission schemes have been proposed,and the transmission stability has been significantly improved compared with the traditional satellite frequency transmission system.In recent years,in order to further improve the transmission stability,most schemes increase the frequency of the radio frequency(RF)signal in exchange for higher detection and compensation accuracy.However,these schemes usually require a frequency division down-conversion method to convert high-frequency signals into low-fr-equency atomic clock frequency signals(100MHz or 10MHz)to be compatible with the synchronization frequency of most terminals,which is convenient for users.In this paper,we propose a novel fiber-based stable frequency transmission scheme,which achieves a high detection and compensation accuracy and realizes long-distance steady phase transmission of the low frequency signals.However,it takes into account system performance and user requirements.The main research contents and results of the paper include:1)For the problem of directly transmitting low frequency RF signals required by users,the error signal detection and compensation accuracy are low.A oscillation frequency multiplier based on the phase-locked frequency multiplication mothed is used to multiply the low-frequency signal at the near end.It can obtain a high frequency signals and transmit them to achieve the high-precision detection and compensation of the transmission error,and the stable low-frequency atomic clock signal is obtained by the frequency mixing mothed at the far end.2)In the transmission process,the deterioration of the optical signal-to-noise ratio and the optical power jitter would limit the transmission distance.For this problem,a low-noise bidirectional optical amplify module is designed to ensure the signal-to-noise ratio and power stability of the optical signals carrying RF signals in ultra long distance fiber.It will reduce the impact of optical signal-to-noise ratio degradation and optical power jitter on transmission stability.3)The stable transmission of 2.4GHz,2.3GHz and 100MHz signals over the 1007km ultra long-haul fiber link is achieved.The results show that the allan deviation of the low-frequency stable transmission system is 1.2×10-13/s and 5.1×10-16/20000s.The factors that limit system performance are discussed.