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Choosing the Right Timing Discriminator for the Application
Fast timing discriminators are useful in two different applications:
a) counting narrow pulses at very high counting rates, and b)
precisely marking the arrival time of these same pulses. Fast timing
discriminators are designed to achieve the best time resolution and
the highest counting rates by operating on the fast-rising detector
signal. Somewhat worse time resolution at much lower counting rates
is available by applying timing single-channel analyzers to the slow
output pulse from a linear pulse-shaping amplifier. This slow timing
solution is described in the Single-Channel
Pulse-Height Analyzer introduction. The fast timing solution is
discussed here.
Historically, fast timing discriminators were designed to work with
negative pulses from the photomultiplier tube anode fed directly to
the discriminator on a terminated 50-Ω
coaxial cable. Consequently, a negative input polarity on a 50-Ω impedance has become the standard. Following this
convention, "rise time" refers to the time taken to make the
transition from 10% to 90% of the pulse amplitude on the leading
edge of the pulse, and "fall time" specifies the transition time
from 90% to 10% of the amplitude on the trailing edge of the pulse. |
