Research On The High Precision Time Measurement System For The LHAASO Project

To explore the origin of cosmic rays, all sky survey for gamma ray sources withvery wide field of view and high sensitivity becomes an important means. Therefore ourscientists have proposed to construct a Large High Altitude Air Shower Observatory(LHAASO) at an altitude of4400m a.s.l..In order to obtain the inject direction ofcosmic rays with an angular resolution of0.5°, the arriving time of shower particlesshould be precisely measured by a ground detector array spread over1.2km2(KM2A),and a timing accuracy of500ps (RMS) for5635frontend electronicsunder a widetemperature range should be achieved. Under this background, the high precision timemeasurement system for the LHAASO project is studied.All nodes of a large scale time measurement system should obtain synchronousclocks with high precision and accuracy. AnEthernet-based method called White Rabbit(WR) is capable of providing synchronized clocks for multiple nodes over a large areawith sub-nanosecond accuracy. The White Rabbit principle and synchronization modelare introduced, and the key parameters of this model are calibrated and verified byexperiments.The temperature effects on all elements of the synchronizationmodel areanalyzed, and their contributions to the final performance are quantified. Thetemperature effects on the WR hardware routing delays are calibrated and automaticallycorrected. The timing performancesof the White Rabbit under different applicationenvironments are tested and analyzed.High precision Time-to-Digital Converter (TDC) is essential forall nodes toachieve high timingmeasurementperformance. The basic principle and design method ofFPGA carry chain based TDC is introduced. The factors that affect the TDC precisionare analyzed, including the carry chain structure, temperature and core voltage. Anautomatic correction method is developed for the temperature effect of FPGA carrychain. A multi-channel TDC is implemented in a low cost FPGA of Cyclone II series,and a one-shot measurement precision of23ps (RMS) is achieved under a widetemperature range.Based on the methods above, aprototype of the high precision time measurementsystem for the LHAASO project is designed.According to the application mode ofcosmic ray observatories, the LHAASO data transfer and clock synchronization network topologyis designed by integrating data and timing linksand providingredundant connections. The hardware and firmware of the time measurement node areaccomplished with modular design method. The timing performance is tested underdifferent environments and analyzed. According to the test results, the timingaccuracycan be estimated to be better than1nswhen5layers of WR switches arecascaded and the temperature span is40oC, and the timing precisionunder any certaintemperature is better than65ps.This timing performancemeets the requirements of theLHAASO-KM2A and most cosmic rayobservatories.