Precise Clock Synchronization in the Readout Electronics of WCDA in LHAASO

TL;DR

This paper presents an enhanced White Rabbit-based method for achieving sub-50 ps clock synchronization precision in the LHAASO WCDA readout electronics, ensuring high timing accuracy over long fiber links.

Contribution

An improved White Rabbit synchronization scheme is proposed to automatically align clock phases and compensate for temperature-induced delays in large-scale detector electronics.

Findings

Achieved synchronization precision better than 50 ps.

Demonstrated stable clock synchronization over 1 km fibers.

Validated the method with initial test results in prototype electronics.

Abstract

The Water Cherenkov Detector Array (WCDA) is one of the key parts in the Large High Altitude Air Shower Observatory (LHAASO). In the WCDA, 3600 Photomultiplier Tubes (PMTs) and the Front End Electronics (FEEs) are scattered within a 90000 m2 area, while a time measurement resolution better than 0.5 ns is required in the readout electronics. To achieve such time measurement precision, high quality clock distribution and synchronization among the 400 FEEs (each FEE for 9 PMTs readout) is required. To simplify the electronics system architecture, data, commands, and clock are transmitted simultaneously through fibers over a 400-meter distance between FEEs and the Clock and Data Transfer Modules (CDTMs). In this article, we propose a new method based on the White Rabbit (WR) to achieve completely automatic clock phase alignment between different FEEs. The original WR is enhanced to overcome the clock delay fluctuations due to ambient temperature variations. This paper presents the general scheme, the design of prototype electronics, and initial test results. These indicate that a clock synchronization precision better than 50 ps is achieved over 1 km fibers, which is well beyond the application requirement.