Dead-Time-Related Signal Distortion

For TCSPC with nonreversed start-stop, the end of the dead-time interval is automatically synchronised with the laser pulses. For reversed start-stop this in not the case. The situation is shown in Fig. 7.87. 

Stop Pulse Stop St Pulse Pt Photon op Si Ise Pl dead time ends here op Ise 

The TAC/ADC dead time starts with the stop of the TAG. The end of the dead time ean be at any time within one of the subsequent signal periods. Averaged over a large number of periods, the result is a step in the reeording probability and thus in the recorded waveform. The size of the distortion depends on the ratio of the eount rate to the signal repetition rate and can be estimated as follows. 

The probability, P, to detect a photon in a particular laser period is 

In praetiee the distortion can be minimised in the same way as classic pile-up, i.e. by maximising the signal repetition rate, or, more exactly, maximising the TAG stop rate. The signal repetition rate should be as high as possible, and the setup should avoid frequency division in the reference channel and pulse-by-pulse multiplexing with a common stop pulse (see Sect. 5.5.8, page 117). 

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Fig. 7.87 Dead-time-related signal distortion in high-repetition-rate reversed-start-stop systems. The dead time ends anywhere in one of the subsequent signal periods, which causes a step in the detection probability...

A radieal eure to avoid pulse distortion by dead-time-related counting loss would be to synehronise the end of the dead time with the reference pulses. However, this would result in more mutual influence of start- and stop-related signals and therefore impair the differential linearity of the time measurement. Extending the dead time to a full signal period might also be unacceptable for low-repetition rate experiments. 

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