An Experiment of VLBI Clock-Signal Distribution through Optical Fibers Based on Signal-Delay Measurement Using a Digital Correlator

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 high-precision 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.