Research Of Clock Synchronization And Data Transmission In LHAASO WCDA Readout Electronics

Cosmic rays are high energy particles that come from outer space and mainly consist of charged particles,gamma(γ)photon and neutrinos.Cosmic rays originate in the high-energy activities of celestial bodies and are affected by the space matter and electromagnetic field during transmission.Studying of cosmic rays can help us to understand the property of high energy particles,verify the particle theory and find new particles.And it can also help us to understand the beginning and evolution of the universe.As neutral particles,the y rays are not affected by the space electromagnetic field and can keep its origin direction.So γ ray detection is an important way to find the source of the cosmic rays.Aiming at such targets,the Large High Altitude Air Shower Observatory(LHAASO)is proposed by Chinese scientists.LHAASO is located at an altitude of more than four thousand meters,and it contains several detector arrays,in which Water Cherenkov Detector Array(WCDA)is one of major components.The WCDA consists of three water ponds,covering an area of about 80000 m2,and 3120 Photomultiplier Tubes(PMTs)are placed under water.A distributed and front end digitization architecture is proposed in the WCDA readout electronics design.Compared with the traditional way,which uses long cables to transmit the detector output signals,this architecture can avoid performance deterioration due signal attenuation and noise during transmission,and also high cost of long cables.However,this means that the output data from FEE need to be transmitted over a long distance of 400 to 500 meters to Data Acquisition(DAQ)system.To guarantee the quality of long distance signal transmission,fibers are employed,and in each fiber commands,data,and clock are mixed together to be transferred.Due to high altitude of LHAASO’s location,obvious temperature variation exists over different seasons,and even over the 24 hours within a day,and there is no air conditioning for FEE or fibers.Meanwhile,high resolution time measurement is required in WCDA,so high precision clock synchronization and automatic phase alignment is demanded over a large area.This is quite a challenge and key technique to be work out in the design of WCDA electronics.In this domain,the White Rabbit(WR)method is a promising technique to solve the problem of clock synchronization and phase compensation in case of large areas.However,its reported performance is still limited to sub-nanosecond(ns)level,which can not satisfy the requirement in WCDA.In the work of other laboratory in China and oversea,issues concerning delay variation of electronics itself and fibers are not well systematically addressed in a variable temperature ambience,except that one solution is proposed to use thermometers and Look-Up Tables(LUT)to monitor the temperature and adjust the clock phase.This dissertation focuses on systematical study of the whole clock transmission system,including the characteristics of the electronics and fibers,and the delay increment allocation method is proposed to achieve automatic high precision clock phase alignment among multiple nodes,without the need of thermometers or out peripheral components,which simplifies the system structure,and renders a good flexibility.Besides,studied is also made on the methods for clock phase adjustment via hardware,which is also an important factor influencing the final precision of clock alignment.In WCDA,the data are required to be read out based on the "triggerless"architecture,i.e.all the raw data on the FEE are transferred to DAQ and processed by software.In this way,a better trigger flexibility can be achieved.Correspondingly,data transfer speed and reliability becomes a great concern.Therefore,high speed data packaging based on TCP/IP protocol is required.According to the design consideration of the overall LHAASO project,WR switches need to be employed in the WCDA readout electronics,and this means that the clock alignment electronics,TCP/IP packaging logic in the FEE are also required to be compatible with the WR standard,which also constitutes an important task in this paper.The contents of this dissertation are as follows:The first chapter mainly introduces the background of the cosmic rays and the LHAASO experiment.This chapter also presents the requirement for the WCDA readout electronics and the architecture of the electronics,especially the requirements on the clock and data transfer.Chapter 2 describes the clock and data transfer electronics of some large scale physics experiments.And these experiments are good reference in the design of the WCDA readout electronics.Chapter 3 contains two parts.One is the high precision clock phase alignment research and the other is the work on the high speed data transfer based on the TCP/IP standard.Good precision of clock alignment of multiple FEEs has to be achieved in a large area with a varying temperature.Besides,the TCP/IP data packaging also has to be implemented in the FEE.And finally the data and clock are mixed together and transferred via the same fiber.Chapter 4 presents the implementation of the data and clock transfer electronics in detail,including the hardware circuit and the logic in the Field Programmable Gate Array(FPGA)device.Chapter 5 presents the system test results.Several types of tests,with one single layer or multiple layers of WR switches in constant or varying temperature ambiences were conducted to evaluate the clock phase alignment performance,and jitter tests of the clock signal were also conducted.As for data transfer,data rate test and BER(Bit Error Rate)tests were conducted in two cases-with only one single FEE and multiple FEE connected to one switch together.The chapter 6 summarizes this dissertation prospects the future work.