The analog signal processing system for the Auger fluorescence detector prototype

The fluorescence detector of the Pierre Auger Cosmic Ray Observatory will provide a measurement of the parameters of extended air showers in the range from 10¹⁹ to 10²¹ eV. An array of 20×22 photomultiplier tubes (PMTs) is placed at the focal surface of a large-aperture telescope thus forming one of the 30 detector modules. The shape of the signal generated by each PMT is variable, depending mostly on the geometry of the air shower as seen by the detector; after analog processing the waveforms will be sampled at a rate of 10 MHz with 12 bit resolution. We have developed an analog signal processor to achieve the best compromise between energy and time resolution, low noise, and low cost. The head electronics provides an active bias network for the PMTs, which keeps the gain constant even in the presence of large dc background light from the night sky. This dc level is measured by means of a built-in optocoupled linear circuit. The pulse signal is sent through a twisted pair to the analog front-end board. At this stage a compression of the 15-bit dynamic range of the signal into the 12-bit range of the FADC is performed. Antialiasing is provided by a Bessel filter