Study On Fiber-Optic Radio Frequency Transfer Based On The PLL Technology

In recent years,due to the development of science and technology,the stability of atomic clock has been greatly improved.Now atomic clock stability can be up to the order of 10^-16/day,which demands for ultra-stable frequency transfer technology.The traditional transmission schemes based on GPS are vulnerable to interference of the free-space channel,and can only reach the stability of 10^-15/day,which cannot meet the requirements of the high stable frequency transfer.Due to the advantages of fiber with low loss,anti-interference,high bandwidth,optical frequency transfer over fiber has attracted growing research interest.When using optical fiber to transfer radio frequency?RF?,the background noise is introduced by the electrical and optical devices.Besides,optical fiber will be affected by external environment such as temperature,which will introduce phase noise.These additional noise works on the RF signal,which will deteriorate its stability.Therefore,in order to achieve high stable fiber RF transfer,the corresponding phase compensation technology need to be studied and adopted.In the passive schemes,the phase noise compensation techniques are mainly based on the mixing structure,with advantages of the simple structure,fast compensation speed,and infinite dynamic range.However,this compensation method usually requires multi-stage circuit mixing and auxiliary microwave sources,which increases the price of compensation system and deteriorates the stability of the received RF signal.The active phase noise compensation schemes based on the phase-locked loop?PLL?have been studied and applied early,due to its many advantages,such as fast compensation speed,large compensation bandwidth,and mature PLL technology.However,the existing PLL schemes usually cannot meet the requirements of both guaranteeing the bidirectional propagation symmetry and suppressing backscatter noise at the same time.To solve the problem,we propose a fiber-optic RF transfer scheme based on PLL phase noise compensation adopting a carrier suppressed double-sideband?CSDSB?signal.The forward and backward signals are transmitted over the same fiber with the same wavelength to guarantee the bidirectional propagation symmetry.Besides,the impact of backscattering is efficiently suppressed by electrical filtering at sites.The main work of this paper includes:?1?The existing fiber-optic RF transfer schemes are introduced andanalyzed.The existing problems in the compensation schemes based onPLL and the distributed RF transfer schemes are elaborated in detail.?2?The noise in fiber-optic RF transfer link is introduced,and theinfluences of the laser,backscattering and circuit devices on RF stabilityare analyzed carefully.?3?A fiber-optic RF transfer scheme based on PLL phase noisecompensation adopting a CSDSB signal is designed and analyzed,andthe expanded schemes adopting passive compensation method is alsointroduced.?4?The principle of the phase noise testing is introduced and thecorresponding circuit are produced.Based on this,the relevantexperiments of the proposed RF transfer via 40km optical link arecompleted.The results show that the proposed scheme can reach thestabilities of 3.9e-14/s and 2.7e-16/10000s at the remote user node andmidpoint of the 40km link.