Trigger Cell

Fast shaped signals with a shaping time of 200 ns from each counter are generated in Shaper/QT (SHQT) and merged into a trigger cell (TC) composed of adjacent 4 $\times$ 4 crystals that are the minimum unit for the ECL trigger system. A gain adjustment of 3-bits on each TC is performed to compensate for different light output of each crystal. Each TC is manipulated on a special trigger circuit board called sum-trigger module (STM) and a TC signal above a certain threshold energy from 50 to 100 MeV depending on trigger conditions is recognized as a hit cluster. The rms variation of threshold values among TCs is $\sim$ 3$ and has been stable during real data taking runs. Furthermore, a constant fraction discriminator is adopted in order to minimize the time-jitter of the trigger signal and to minimize its time spread. In the actual experimental situation, the noise level of TC units was measured to be about 8 MeV and the rms timing resolution less than 20 ns. The latter value should be compared with a long decay constant () of CsI(T$l$) scintillation light. An analog-sum signal and a logic signal above the threshold from each TC provide the total energy trigger and the cluster counting trigger, respectively.