Abstract

Long-distance radio frequency (RF) synchronization through fiber link is attracting more attention in recent years. The repeater becomes increasingly important with the increase of the transmission distance. In this article, we analyze the three main relay methods principally including single-span, signal relay, and cascade connection frequency transmission and compare their frequency instability. The proof-of-concept experiments are also conducted along a 3009.8 km fiber link in the laboratory environment. The system used for comparison is a combination of phase conjugation and phase-locked loop (PLL) designed for long-distance frequency transmission. The best transfer preference can be obtained by using the cascade connection, whose frequency instabilities of $8.8 \times {10^{ - 14}}$ @ 1 s and $8.4 \times {10^{ - 17}}$ @ 10 000 s have been measured for phase-stable RF signal. Our study can be useful for the calibration and comparison of ultra-long-distance atomic clocks.