A prototype clock system for LHAASO WCDA

In this paper, a prototype clock system with high precision for Water Cherenkov Detector Array (WCDA) in Large High Altitude Air Shower Observatory (LHAASO) project is presented. In WCDA, a distributed architecture that digitalization is implemented nearby detectors in the front is imperative because the detectors are 150m far from the counting room. However, in this architecture, how to provide an accurate synchronization clock of stable phase for all the front-end electronics (FEE) is not an easy task. A prototype clock system consisting of clock source module, clock transmitting modules and clock receiving modules is developed to achieve this aim. Based on Serializer/Deserializer (SerDes) and fiber transmission, the system clock generated by the clock source module, locked to the global synchronous clock, is transmitted to all the FEEs. Referred to White Rabbit project, a reduced and improved scheme is presented to automatically adjust the propagation delay and keep the phase alignment. In the scheme, an FPGA TDC whose resolution is 1.25ns is presented to measure the phase difference zoomed in by digital Dual Mixer Time Difference (DMTD) technique. With the help of direct phase-shifting mode of FPGA's DCM, the phase can be dynamically and repetitively moved forward and backwards by the value of one DCM_TAP which is less than 40ps. The experimental results show that both the clock jitter (RMS) and clock skew of all the FEEs are less than 100ps, which meet the requirement of LHAASO WCDA. It has been proved that this prototype clock system can be used in the whole WCDA electronics. Furthermore, this work provides a general approach to develop high quality synchronous clock systems for front end electronics in large physics experiments.