TOF Trigger System

The TOF trigger system provides an event timing signal and information on the hit multiplicity and topology to GDL. The multiplicity and topology information can be used to perform internal event selection and reduce the rate of timing signals to acceptable levels before it is delivered to GDL. A prompt Level-0 (L0) trigger signal is also delivered to SVD based on independent event selection criteria.

The timing signal of an event is required to be sufficiently precise to make a gate for the ECL readout and to provide $T_0$ to the CDC readout, namely to have a time jitter less than 10 ns. Owing to the very fast time response of TOF signals, a time jitter of about 5 ns is feasible. Although the timing of each PMT signal varies in a time range of 4$\sim$33 ns depending on the hit position, the mean time stays within a small time range of 4.8$\sim$7.2 ns for the Belle TOF configuration. The time jitter caused by variations in pulse height (time walk effect) is expected to be less than 1 ns at the nominal discrimination level. Thus the TOF mean time can provide a precise event timing signal with a jitter of about 5 ns. This signal must be delivered at rates below 70 kHz to ensure only one timing signal to appear in the GDL event window.

The SVD readout electronics calls for a shaping time faster than $\sim$ 2 $\mu$s required by GDL to make an event decision. The TOF readout and trigger electronics can provide a prompt trigger signal with approximately a 0.85$\mu$s delay. This includes $\sim$500 ns to make internal event multiplicity and topology calculations used to reduce the level zero rate to acceptable levels. The trigger rate must be below 40 kHz to ensure a reasonable deadtime in the SVD readout electronics.