Prototype of the Readout Electronics for WCDA in LHAASO

In the large high altitude air shower observatory, the water Cherenkov detector array (WCDA) is one of the key parts. The WCDA consists of 3600 photomultiplier tubes scattered in a 90 000-m(2) area, and both high precision time and charge measurements are required over a large dynamic range from 1 to 4000 photoelectrons (P.E.). To achieve time measurement precision better than 500-ps rms, high quality clock distribution and automatic phase compensation are needed among the 400 front-end electronics (FEE) modules. To simplify the readout electronics architecture, clock, data, and commands are transferred simultaneously over 400-m fibers, while high speed data transfer interface is implemented based on TCP/IP protocol. Design and testing of the readout electronics prototype for WCDA is presented in this paper. Test results indicate that a charge resolution better than 10% rms at 1 P.E. and 1% rms at 4000 P.E. and a time resolution better than 300-ps rms are successfully achieved over the whole dynamic range, beyond the application requirement.