Development of Digital Readout Electronics for the CMS Tracker

The Compact Muon Solenoid (CMS) is a general-purpose detector, based at CERN in Switzerland, designed to look for new physics in high-energy protonproton collisions provided by the Large Hadron Collider. The CMS tracker has 10 million readout channels being sampled at a rate of 40 MHz, then read out at up to 100 kHz, generating huge volumes of data; it is essential that the system can handle these rates without any of the data being lost or corrupted. The CMS tracker FED processes the data, removing pedestal and common mode-noise, and then performing hit and cluster finding. Strips below threshold are discarded, resulting in a significant reduction in data size. These zero suppressed data are stored in a buffer before being sent to the DAQ. The processing on the FEDs is done using FPGAs. Programmable logic was chosen over custom ASICs because of the lower cost, faster design and verification process, and the ability to easily upgrade the firmware at a later date. This thesis is concerned with the digital readout electronics for the CMS tracker, working on the development of the FED, and verifying that it will meet the requirements of the detector. Firmware was developed for the back-end FPGA of the FED, implementing the CMS-wide common data format. Each event is wrapped in a header/trailer containing information such as the trigger number, bunch-crossing number and error-detection information. The firmware was developed in VHDL, and will be incorporated into the back-end FPGA of the FED, both in the tracker, and in the other subdetectors. A study was performed, looking into the flow of data and buffer levels in the FED. A program was written in C++ that models the behaviour of the FED buffers. It was shown that during normal operation the FED can handle occupancies of up to 4 % with 100 kHz random triggers, assuming a sustainable output rate of 200 Mbyte/s. Even when the trigger rate was increased to its maximum of 140 kHz, the FED buffers did not overflow as long as the occupancy remained below 2.5 %. These results confirmed that the FED buffers should never overflow in normal operating conditions.