Leading-edge discriminator

Of course, a simple leading edge discriminator cannot be used to trigger on such pulses. The amplitude jitter would introduce a timing jitter of the order of the pulse rise time (Fig. 4.1, left). In practice the timing jitter is even larger because any discriminator has an intrinsic delay that depends on the signal slope speed and the amount of overdrive. 

In practice, direct triggering at a signal level of exaetly zero is impossible. Ultrafast discriminators tend to oscillate at zero input voltage, and the smallest noise amplitudes outside a PMT pulse cause the discriminator to trigger. Therefore, practical implementations of the principle use a combination of a leading edge discriminator and a zero eross trigger. The principle is shown in Fig. 4.2. 

The comparator of the leading edge discriminator, Cl, is connected directly to the input and responds when the input voltage exceeds a defined threshold. 

Figure 4.17. Constant fraction discrimination in TCSPC. Tbp Timing error due to puUe bei variations using leading-edge discrimination. Bottom Operation of a constant fraction discriminator.

The leading-edge timing method determines the time of arrival of a pulse with the help of a discriminator, as shown in Fig. 10.20. A discriminator threshold is set and the time of arrival of the pulse is determined from the point where the pulse crosses the discriminator threshold. ... 

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