Improvement of nonlinearity correction for BES III ETOF upgrade

An improved scheme to implement integral nonlinearity (INL) correction of time measurement for BES III endcap TOF (ETOF) upgrade system is presented in this paper. Methods for INL correction in HPTDC (high performance time to digital convertor) adopted in BES III TOF system are proved to be limited in upgrading MRPC system, because sharply increased electronic channels corresponding to detector read-out strips degrade system configuration efficiency severely. Besides, the function of system-level nonlinearity determination is not involved in BES III TOF electronics after installation, which ignores the possibility to monitor INL of TDC in spectrometer scene. In this proposed method, during INL correcting procedure, the INL data is automatically imported from a non-volatile storage of read-only memories (ROM) instead of from DAQ, and then fed into FPGA with correction algorithm online. The realtime performance and system efficiency for INL correction can be guaranteed with the help of this method. Based on a dedicated on-board reference clock, a start signal irrelevant to 41.65 MHz clock generated from BEPC II is sent to FEE to produce a test signal that finally serves as pseudo-random pulse to HPTDC for nonlinearity evaluation. Test results show that the INL errors for one time measuring module with 72 channels can be corrected online and in real-time. The measuring and DAQ systems simplification and stability can be guaranteed as well, which makes this method successfully meet the requirement for BES III ETOF upgrade phase. Furthermore, the system-level nonlinearity test is also supported.