Cathode Trigger Comparator

The CMS Level-1 trigger system is designed for a maxim um output rate of 100 kHz and single muon trigger rate is capped at few kHz. Studies (CMS TN/94-261) showed that, for p_t thresholds of 20-40 GeV/c, a 30% momentum resolution is required to achieve this limit. For rapidity < 2.4, Monte Carlo calculation demonstrates that 30% momentum resolution is achievable if track hits can be localized to within half the strip width. The method adopted in the baseline design to realize this trigger resolution involves the comparator network (CMS TN/94-212) scheme. The basic building block of the comparator network is shown in this figure.

Four comparators are used for every input channel. The pulse from the fast shaper for strip n is compared with a pre-set threshold level. If the pulse exceeds threshold, then after a short time delay to allow signals to peak, the voltage for strip n is compared with pulses from neighboring strips (strip n-1 and strip n+1). Strip n has the peak charge if its pulse is larger than the neighbor strips. At the same time, the track hit position is localized to either right or left half of strip n by a fourth comparator which compares pulses from strip n-1 and strip n+1. The output levels from the comparators are latched synchronously with the bunch crossing frequency and fed into AND gates to produce two digital signals, Ln and Rn, per strip. The Ln and Rn signals are used for the high Pt (10-100 GeV/c) trigger where good spatial resolution is important. Additionally, these signals are ganged 4:1 into 2-strip wide bits for use by a low Pt (2.5-10 GeV/c) trigger. Since the analog signals are slow compared to the bunch crossing interval, the five comparator network output signals (four half-strip plus one ganged output) are time-multiplexed into three consecutive bunch crossings for output from the comparator chip. In the first bunch crossing, an output bit represents a hit on either of the strips, while during the following two crossings the output contains encoded information as to the half-strip location of the hit.