为开展长距离高精度时间频率传输系统研究，对多级光纤时间频率传递级联系统的时延抖动进行理论分析，得出级联系统总时延抖动的一般公式。在实验上实现了两级级联50 km光纤时频传递系统的闭环锁定，分别通过测试单级系统和级联系统的鉴相误差电压得到其时延抖动。通过实验结果和理论计算比较，分析了主要实验误差来源；通过分析掺铒光纤放大器(EDFA)放大自发辐射(ASE)产生的噪声对信噪比(SNR)的影响，系统频率稳定度由于信噪比的下降而产生劣化。

Based on the theoretical analysis of cascaded optical fiber time and frequency dissemination system, the general formula of total time jitter is calculated. A two-stages optical fiber link for ultra-stable frequency dissemination with a total fiber length of 2×50 km is demonstrated. Then the phase error voltage and time jitter of single-stage and two-stage systems in closed-loop is measured. After comparing experimental results and theoretical calculation, the main sources of experimental error is analyzed. Analyze the impact of phase noise on signal to noise ratio (SNR), which the phase noise is caused by the amplified spontaneous emission (ASE) of the erbium-doped fiber amplifier (EDFA). The stability of transfer system is gotten due to the decline of signal to noise ratio (SNR).