A Prototype Clock System for LHAASO WCDA

In this paper, a prototype of a high precision clock system for the Water Cherenkov Detector Array (WCDA) in the Large High Altitude Air Shower Observatory (LHAASO) project is proposed. Due to the large scale of WCDA, a distributed architecture is required, where the digitalization is implemented near the detectors in front. However, in this architecture, the designers had to face the difficulty of how to distribute the accurate synchronous clock to all the front-end electronics (FEE), with a very stable phase relationship. Thus a prototype clock system was developed, consisting of a clock source module, clock transmitting modules and clock receiving modules. It distributes the system clock to all FEEs, with simultaneous data transmission. Based on the White Rabbit protocol, a reduced and improved scheme is proposed to automatically adjust the propagation delay and keep the phase alignment. In this scheme, the phase difference is zoomed in by the digital Dual Mixer Time Difference (DMTD) technique, measured by a high resolution time-to-digital converter (TDC), and then shifted forward and backwards with the help of the FPGA's Digital Clock Manager (DCM). The experimental results show that both the clock jitter (RMS) and the clock skew of all the FEEs are less than 100 ps.