Dead time, signal distortion

The simplest data reduction algorithm for FIDs consists in averaging the magnitudes of the complex FID signal over a data window positioned within its starting portion (Fig. 26). The window can be freely positioned in a way to cut out any dead-time distortions and, at the same time, minimize field-fluctuation effects. 

Of course, dead-time compensation works with absolute precision only if the photons appear randomly. Unfortunately in most TCSPC applications, the signal is pulsed and most of the photons are detected in only a small fraction of the signal period. Nevertheless, the compensation works well for repetition rates higher than the reciprocal dead time. It can, however, correct for only the loss in the recorded intensity, not for distortion in the recorded pulse shape. 

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...

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. 

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. 

Firstly, the main source of instrumental distortion of the signal is quite different in the two domains. In the CW method, the signal is broadened due to the modulation of the applied RF, and corrections must be made for the effects of this broadening on the different components [30]. In the pulse method, the main distortion of the signal is due to the dead-time. Efforts may be made to minimise the dead-time by optimisation of instrumental characteristics, or in the case of rigid solids, some of the problem may be overcome by the application of suitable echo sequences [18, 37], but it cannot be totally eliminated. 

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