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 eV to 10²¹ eV. An array of 20×22 PMT´s 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 best energy and time resolution, low noise and low cost. The head electronics provides an active bias network for the PMT´s, which keeps the gain constant even in presence of large DC background light from the night sky: this DC level is measured by means of a built-in optocoupled linear circuit