Digital signal filtering system and method of application

A system for enhancing digital signal event counting comprising:

• a front-end amplifier configured to convert current/charge signals collected by a photon detector to voltage signals;
• a plurality of comparators, each being associated with a threshold level, coupled to the front-end amplifier to compare the voltage signals against each threshold level to generate the plurality of comparator outputs at corresponding threshold levels for each voltage signal;
• a pulse filter coupled to the plurality of comparators and configured to generate a refined signal associated with a rising edge and a falling edge of each comparator output, identify timing characterization of at least two refined signals respectively corresponding to a lower threshold level and a next higher threshold level, and generate a trigger signal corresponding to an event counting based on the timing characterization associated with both the rising edge and the falling edge;
• wherein the pulse filter comprises a plurality of filter units coupled to the plurality of comparators in a cascaded configuration with each filter unit receiving the comparator output from one of the plurality of comparators at a corresponding threshold level;
• wherein each filter unit comprises a first-stage circuit coupled with a second-stage circuit, the first-stage circuit configured to process the comparator output to obtain a refined signal and a rising-edge signal and a falling-edge signal to respectively detect the rising edge and the falling edge for differentiating the refined signal;
• wherein the second stage circuit is configured to cross-couple with the first stage circuit coupled to a comparator at the lower threshold level and another first stage circuit coupled to a comparator at the next higher threshold level;
• wherein the second stage circuit comprises a first-branch inverting delay sub-circuit to introduce a first delay to the falling edge of one of two refined signals corresponding to either the lower threshold level or the next higher threshold level to generate a hold signal for discriminating the refined signal corresponding to the lower threshold level with a duration shorter than the first delay if no refined signal corresponding to the higher threshold level is detected within the first delay;
• and a plurality of counters coupled to the pulse filter and configured to increment a count at the lower threshold level or both the lower threshold level and the next higher threshold level for each event counting based on the trigger signal.