Specifications

CFD Inputs 19
Connecting PMTs to the CFD Inputs
PMTs use secondary-electron emission to multiply a single photoelectron by a factor of 10
6
to
10
8
. The output pulses of a PMT are negative, with a average amplitude between 10 mV and
100 mV and a duration between 300 ps and a few ns. Due to the random gain mechanism the
pulse amplitude varies randomly. The variation can be on the order of 1:10. Moreover, there
are a large number of small-amplitude background pulses from the dynode system of the
PMT.
A conventional discriminator would suppress the background pulses, but introduce a timing
jitter on the order of the duration of the leading edge of the pulses. Accurate triggering on
PMT pulses therefore requires a trigger circuit that not only suppresses pulses below a given
amplitude but also triggers at a time independent of the pulse amplitude. A CFD solves the
problem by a double-discriminator principle, see Fig. 24. One discriminator, D1, triggers
when the differentiated pulse crosses the signal baseline. The moment when the differentiated
pulse crosses the baseline is independent of the pulse amplitude. However, because the
threshold of the zero-cross discriminator must be close to the baseline, it also triggers to small
pulses and noise. Therefore a second discriminator, D2, is used to select input pulses above a
given amplitude. Only if the pulse amplitude is larger than a selectable threshold the pulse
edge of the zero cross discriminator is passed to the subsequent timing circuitry. Details of the
CFD design are described in [1, 2].
Zero cross
+
-
Input difference
voltage of D1:
Delay
+
-
Threshold
Input
D1
D2
Zero Cross
Enable
Input pulse
Difference
Threshold
Zero
cross level
Delayed
pulse
D-FF
Fig. 24: Constant-fraction discriminator (CFD)
The threshold of the DPC CFDs can be set from 0 to -500 mV. In principle, all commonly
used PMTs could be connected directly to the CFD inputs. There are, however, reasons why
this is not recommended:
PMTs can deliver pulses of extremely high amplitude. Such pulses can be caused by radioac-
tive particles generating light pulses in the glass of the tube, by instability in the PMT, or by
illumination by strong laser pulses. Although the CFD inputs are overload-protected ex-
tremely strong pulses can damage the sensitive discriminator chips of the CFDs. Moreover,
when the PMT output is connected directly to the CFD there is no control about the output
current of the PMT. However, a PMT can easily be damaged by operation at excessive light
intensities. PMTs should therefore always operated via a preamplifier. The amplifier not only
protects the CFD against damage, it also delivers an overload warning if the maximum safe
output current of the PMT is exceeded. Please see [2, 3] for details.
The recommended operating conditions for a number of PMTs and PMT modules are shown
in the table below.