A fully digital architecture for trigger circuits in programmable logic

A digital trigger module for high-resolution nuclear spectroscopy, composed of three filters operating in parallel and a resolver, is introduced. The single filters are optimized so as to allow low-level discrimination as well as high time-resolving capabilities. Particular care is taken in order to minimize hardware resources. In the case of a high-resolution gamma spectroscopy system based on quasi-coaxial HPGe detectors, the lower threshold discrimination level can be as low as 4 keV while measuring the 1.17-MeV ⁶⁰Co line. The capability of discriminating gamma photons impinging almost at the same time is limited in practice only by the intrinsic charge-collection time of the detector. The circuit is implemented in a programmable device (field-programmable gate array) and exploits at best hardware resources already allocated to the filter for energy estimate. A built-in calibration circuit is also implemented, which allows easy setting of discrimination thresholds. It is shown that the devised architecture can be easily upgraded to accommodate a higher number of parallel filters if a further increase of circuit performance is required.