Commissioning and Calibrating the CMS Silicon Strip Tracker

Abstract

The data acquisition system for the CMS Silicon Strip Tracker (SST) is based around a custom analogue front-end ASIC, an analogue optical link system and an off-detector VME board that performs digitization, zero-suppression and data formatting. A complex procedure is required to optimally configure, calibrate and synchronize the 10 channels of the SST readout system. We present an overview of this procedure, which will be used to commission and calibrate the SST during the integration, Start-Up and operational phases of the experiment. Recent experiences from the CMS Magnet Test Cosmic Challenge and system tests at the Tracker Integration Facility are also reported. I. THE DATA ACQUISITION SYSTEM The CMS Silicon Strip Tracker (SST) is unprecedented in terms of its size and complexity, providing a sensitive area of >200 m and comprising 10 readout channels. Fig. 1 shows a schematic of the control and readout systems for the SST. The control system [1] comprises 300 “control rings” that start and end at the off-detector Front-End Controller (FEC) boards and is responsible for distributing slow control commands, clock and Level-1 triggers to the front-end electronics. The signals are transmitted optically from the FECs to front-end digital optohybrids via digital links, and then electrically via ‘token rings” of Communication and Control Units (CCUs) to the front-end electronics. The readout system is based around a custom front-end ASIC known as the APV25 chip [2], an analogue optical link system [3] and an off-detector Front-End Driver (FED) processing board [4]. The system comprises 76k APV25 chips, 38k optical fibres (each transmitting data from a pair of APV25 chips) and 440 FEDs. The APV25 chip samples, amplifies, buffers and processes signals from 128 channels of a silicon strip sensor at the LHC collision frequency of 40MHz. On receipt of a Level-1 trigger, pulse height and bunch-crossing information from pairs of APV25 chips are multiplexed onto a single line and the data are converted to optical signals that are transmitted via analogue fibres to the off-detector FED boards. The FEDs digitize, zerosuppress and format the pulse height data from up to 96 pairs of APV25 chips, before forwarding the resulting event fragments to the CMS event builder (EVB) and online computing farm. Figure 1: The SST control system uses ∼300 control rings (based around the FEC and CCU boards) to propagate clock, trigger and slow control information to the front-end. The SST readout system is based around the APV25 chip, an analogue optical link system and the off-