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时频参考信号的传输距离远,环境温度变化造成信号延迟波动较大,精度要求极高的毫米波段VLBI观测需要对传输延迟进行精确测量与补偿。本文根据相关机延迟测量法对VLBI时频信号光纤传输中的信号延迟进行了测量试验,采用高性能相关机直接测量往返信号的相位差来获得信号的传输延迟,测试结果表明该方法能达到亚皮秒级传输延迟测量精度,系统稳定、可靠。

In a VLBI observation delays of clock signals used as timing and frequency standards fluctuate severely after long-distance propagation from a maser to a radio receiver． The fluctuations are worsened by variations of environmental temperatures． There is a need of a system to measure and compensate clock-signal delays with high precisions for a VLBI observation in the millimeter wavelength band． In this paper we present a system for the purpose． In the system clock signals are distributed through optical fibers and the system is based on signal-delay measurement using a digital correlator． With the high-performance digital correlator highprecision results of signal delay can be obtained by directly measuring phase differences between signals transmitted to and back from the optical receiver． Our test results show that the system can achieve measurement precisions better than the level of a picosecond，and the system is highly stable． Our test thus verifies the overall reliability of the system．