The Large Area Water Cherenkov Array （LAWCA） experiment focuses on high energy gamma astronomy between 100 GeV and 30 TeV. Invoked by the idea of hardware triggerless structure, a prototype of LAWCA trigger electronics is implemented in one single VME-9U module which obtains all the data from the 100 Front End Electronic （FEE） endpoints. Since the trigger electronics accumulate all the information, the flexibility of trigger processing can be improved. Meanwhile, the dedicated hardware trigger signals which are fed back to front end are eliminated; this leads to a system with better simplicity and stability. To accommodate the 5.4 Gbps system average data rate, the fiber based high speed serial data transmission is adopted. Based on the logic design in one single FPGA device, real-time trigger processing is achieved; the reprogrammable feature of the FPGA device renders a reconfigurable structure of trigger electronics. Simulation and initial testing results indicate that the trigger electronics prototype functions well.
Due to the large scale of Water Cherenkov Detector Array in Large High Altitude Air Shower Observatory, the frontend digitization is imperative.Thus a clock distribution system is desired,which broadcasts the synchronous clock signals with low jitter to the frontend electronics distributed in the field of 90 000 m^2.The White Rabbit protocol provides an option,which has been approved to achieve sub-ns accuracy and ps jitter in the synchronization of around 1 000 nodes in the order of 10 km.But the hierarchy of the original is too complex for Large High Altitude Air Shower Observatory application.Thus we proposed a reduced scheme based on the White Rabbit protocol.The validation circuit shows that the clock skew due to the fiber length difference can be adjusted to less than 25 ps and the clock jitter is less than 62 ps.
In the readout electronics of the Water Cerenkov Detector Array （WCDA） in the Large High Altitude Air Shower Observatory （LHAASO） experiment, both high-resolution charge and time measurement are required over a dynamic range from 1 photoelectron （P.E.） to 4000 P.E. The Analog Front-end （AFE） circuit is one of the crucial parts in the readout electronics. We designed and optimized a prototype of the AFE through parameter calculation and circuit simulation~ and conducted initial electronics tests on this prototype to evaluate its performance. Test results indicate that the charge resolution is better than 1%@4000 P.E. and remains better than 10%@1 P.E., and the time resolution is better than 0.5 ns RMS, which is better than the application requirements.
The LHAASO （Large High Altitude Air Shower Observatory） experiment is proposed for a very high energy gamma ray source survey, in which the WCDA （Water Cherellkov Detector Array） is one of the major coinponents. In the WCDA, a total of 3600 PMTs are placed under water in four ponds, each with a size of 150m×150 m. Precise time and cimrge measurement is required for the PMT signals, over a large signal amplitude range from a single P.E. （photo electron） to 4000 P.E. To fulfill the high requirement of a signal measurement in so many front end nodes scattered in a large area, special techniques are developed, such as multiple gain readout, hybrid transmission of clocks, commands and data, precise clock phase alignment and new trigger electronics. We present the readout electronics architecture for the WCDA and several prototype modules, which are now being testedin the laboratory.
The Water Cherenkov Detector Array （WCDA） is one of the core detectors in the Large High Altitude Air Shower Observatory （LHAASO）, and it consists of 3600 photomultiplier tubes （PMTs）. Both high resolution time and charge measurement are required over a large dynamic range from 1 photoelectron （P.E.） to 4000 P.E. The prototype of an analogue front-end Application Specific Integrated Circuit （ASIC） fabricated using Global Foundry 0.35 μm CMOS technology is designed to read out the PMT signal in the WCDA. This ASIC employs leading edge discrimination and an （RC）4 shaping structure. Combined with the following Time-to-Digital Converter （TDC） and Analog-to-Digital Converter （ADC）, both the arrival time and charge of the PMT signal can be measured. Initial test results indicate that time resolution is better than 350 ps and charge resolution is better than 10% at 1 P.E. and better than 1% with large input signals （300 P.E. to 4000 P.E.）. Besides, this ASIC has a good channel-to-channel isolation of more than 84 dB and the temperature dependency of charge measurement is less than 5% in the range 0 50℃.
