Optimization of the pulse arrival time determination in the ZEUS central tracking detector FADC system

The constant fraction discriminator algorithm for finding the arrival time of pulses from a drift chamber is outlined. Using a Monte Carlo simulation the effect of noise and pulse rise time on the timing resolution is studied. A simple correction to this algorithm, which reduces the effect of non-linearities in the pulse rising edge is described. The effect of the correction on the ZEUS central tracking detector, instrumented with an eight-bit flash analogue to digital converter system is studied. Using calibration pulses the timing resolution of the system is measured to be 0.021 ± 0.003 digitizing periods, which is in agreement with 0.024 ± 0.001 predicted by the Monte Carlo simulation. Without the correction the resolution is 0.17 ± 0.02 digitizing periods. The optimum risetime for the ZEUS system is estimated from this model. The model is applicable to other systems which require accurate timing of pulses and have a relatively constant pulse shape.