The CMS Fast Beam Condition Monitor for HL-LHC

The high -luminosity upgrade of the LHC brings unprecedented requirements for realtime and precision bunch -by -bunch online luminosity measurement and beam -induced background monitoring. A key component of the CMS Beam Radiation, Instrumentation and Luminosity system is a stand-alone luminometer, the Fast Beam Condition Monitor (FBCM), which is fully independent from the CMS central trigger and data acquisition services and able to operate at all times with a triggerless readout. FBCM utilizes a dedicated front-end application -specific integrated circuit (ASIC) to amplify the signals from CO2-cooled silicon -pad sensors with a timing resolution of a few nanoseconds, which enables the measurement of the beam -induced background. FBCM uses a modular design with two half -disks of twelve modules at each end of CMS, with four service modules placed close to the outer edge to reduce radiation -induced aging. The electronics system design adapts several components from the CMS Tracker for power, control and read-out functionalities. The dedicated FBCM23 ASIC contains six channels and adjustable shaping time to optimize the noise with regards to sensor leakage current. Each ASIC channel outputs a single binary high-speed asynchronous signal carrying time -of -arrival and time -over -threshold information. The chip output signal is digitized, encoded, and sent via a radiation -hard gigabit transceiver and an optical link to the back -end electronics for analysis. This paper reports on the updated design of the FBCM detector and the ongoing testing program.