Abstract: Background: General Purpose Powder Diffractometer (GPPD) is one of Chinese Spallation Neutron Source (CSNS) diffractometers, which adopts Symptom Scale of Neurotransmitter Deficiency (SSND) detecting the position and the flight time of neutron when it hits, and the electronics system processes the detected neutron information. The shooting time T0, as a time reference point of the electronics system, is a real time signal. Traditional cable transmission meets the real-time signal transmission requirements, however, laying work will be rather complex and bring great difficulties to project since numerous of cable was needed. Purpose: The electronics control circuits is needed to transfer the received signal T0 to the lower electronics in real time with low propagation delay and low jitter via optical fiber, and the the propagation delay is less than 1 μs which including the 15-m optical fiber delay, the jitter is less than 0.1 μs. Methods: This paper describes a transferring scheme based on Spartan6 XC6SLX75T FPGA using Aurora protocol via optical fiber. The testing program is based on ISE Design Suite14.4 FPGA development platform, adopting high-precision oscilloscopes to the measure and calculate the transmission delay and jitter of different rise time signal T0 repeatly. Results: Repeated measurements show that the maximum propagation delay is less than 600ns and that the maximum jitter is less than 4 ns, in addition, the stability of jitter stability is good, and it meets the system requirements. Conclusion: It is feasible to use Aurora protocol via optical fiber to transfer real-time signal T0. Signal rise time has a certain influence on the delay and jitter, longer signal rise time corresponds longer delay and greater the jitter, so to shorten rise time of signal can reduce the transmission delay and jitter.

Key words: Chinese Spallation Neutron Source (CSNS), General Purpose Powder Diffractometer (GPPD), Real-time signal, Jitter, Aurora protocol, Optical