Development of a clock distribution system for sub-nanosecond time synchronization over long distances

The Askaryan Radio Array (ARA) is a new detector deployed at the South Pole designed to detect ultrahigh-energy neutrinos using radio frequency signals emitted by neutrino-induced cascades in the glacial ice. The whole array will contain 37 stations which cover O(100) km
²
surface area. Each station consists of four two-hundred-meter-deep holes spaced 20 meters apart with 4 antennas in each hole at depths ranging from 180 - 200 m. A custom-designed ASIC nominally located in close proximity to the antennas is used for high-speed digitization of the induced RF signals. In order to perform the complex particle reconstructions, each antenna signal must be recorded with a time precision of 50 ps relative to other antennas in the same station. In addition the digital data stream from the digitizer must be transmitted from the hole bottom to logic on the surface. This note describes our group's solution to both challenges that uses commercially available high-speed transceivers and the clock data recovery functions built into these ICs. This includes both a CAT5E twisted pair version and an optical fiber version. While this application is discussed in particular, the technology has potential applications in many fields: any system that requires ultra-high precision synchronization of two or more remote clocks could benefit from the system described herein.