The NEMO experiment data acquisition and timing distribution systems

Many high energy physics experiments (Auger, Ice-Cube, Km3Net) currently taking data or planned in the near future consist of big and sparse detectors that acquire a huge amount of data. This kind of topology requires the ability to acquire physics information in different places of the detector and correlate them using the occurrence time, usually with a sub-ns precision. In some cases this can be done using commercially available technologies, for instance the Auger experiment uses a GPS receiver in each surface tank to “time stamp” the produced data, but in many cases this is not possible. In underwater or underground detectors the GPS signal cannot reach the apparatus so a different approach must be used. In order to minimize the number of physical links and to guarantee the timing synchronization of the apparatus, data and clock information can be transmitted over the same serial stream. This work presents a synchronous link with fixed and deterministic latency based on the embedded transceivers of the Xilinx Virtex-5 FPGA family. This communication channel is used to implement the data acquisition system for the NEMO experiment. The whole timing distribution chain has been taken into consideration to assess the performance of the readout electronics in terms of timing resolution. The system clock distributed at every level of the apparatus has been derived by a precise timing generator during the characterization phase and by the signals provided by a GPS receiver during the operating configuration. The overall timing performance comply with the requirements of the experiment, yielding a rms timing resolution of about 20 ps.