Automatic pole-zero/zero-pole digital compensator for high-resolution spectroscopy: design and experiments

In a high-resolution spectroscopy system the relatively long exponential decay due to the charge preamplifier is customarily canceled in an analogue fashion by means of a PZ (pole-zero) stage. The accurateness of such a compensation has a big impact on the energy resolution because it strongly affects the baseline-stability problems. Automatic methods operating in the analogue domain are used but they require mechanical adjustment of trimmers, which is bulky and not adequate in experiments with many measurement channels. We have automatically and on-line performed such a compensation in a digital way, while maintaining a spectroscopy performance and keeping at minimum both the ADC sampling frequency (thus power consumption) and its resolution (thus cost). This is done through an IIR filter, implemented within a FPGA by a DSP. The so-compensated waveform has, in excellent approximation, an all-pole shape. Starting from such a signal, the minimum-noise filters for energy and/or time measurements are then promptly synthesized and implemented for real time operation through the same DSP.