Electronics of BESIII TOF monitor system

This paper describes the control electronics for the TOF (Time of Flight) monitor system of BESIII. A 9U VME module is designed as an interface between the TOF monitor laser box and BESIII data acquisition (DAQ) system. It enables the laser source, sends trigger signals to the laser driver, switches the optic path, and measures the charge and arrival time of pulses from two reference phototubes (Ref. PMT) as well. After sending each trigger pulse to the laser source, the monitor electronics module also sends a L1 (Level 1) trigger to the TOF readout system. The whole interface and control logic is implemented in an Altera FPGA (Field Programmable Gate Array) mounted on the Monitor module, and signals from the Ref. PMTs of the laser system are digitized with the CERN HPTDC technique. According to a serial of experimental tests, the time resolution of monitor electronics is better than 25 ps, and the relative methodical error of charge measurement is less than 0.5% after quadratic fitting. The TOF monitor system was installed at the end of 2007, and has been operated continuously since then. A total time resolution of about 70 ps and a charge resolution of about 4% of the TOF counters have been obtained during field monitor tests.