Page 22 of 25
The Constant Fraction Discriminators CFD8c, CFD7x, CFD4c, CFD1c and CFD1x (11.0.1701.1)
Changing the
Width Offset
potentiometer is only recommended if a very short dead-time is needed (see Figure 3.27b). It is
important to note that the dead time of the
CFDx
circuit is significantly larger than the CFD dead time for signal timing only.
If two signals arrive within 50 ns the pulse height information may be mixed.
Figure 3.27b: as Figure 3.25b right, but including the
CFDx
output (lowest trace) for three different setting of the
“width offset” poti. The pulse-height/delay ratio is not affected by changing this setting, only the delay offset at
zero pulse height. The middle picture shows a typical setting, in the left picture the setting is optimized for low
dead-time (requires a multi-transition TDC with low dead time). In the right picture the offset is set too high,
indicated by a flat base at the end of the signal (marked in red): The circuit will not function properly in this
condition
Figure 3.28b shows a pulse height distribution spectrum that has been acquired by measuring the relative time between the
stop
signal and the CFD timing signal with a
RoentDek
TDC8HP
. The same spectrum could be recorded by plotting the time
difference between trailing and leading transitions of the
CFDx
output. It is advisable to mark the pulse height zero position
by retracting the cable on the
A
in
socket for a few seconds (see red arrow). This offset changes as function of the
Width Offset
potentiometer setting and cable lengths (e.g. in case of
stop
output use).
Figure 3.28b: Lin and log plots of the pulse height distribution from a micro-channel plate as obtained
with a
CFD1x
unit. The peak for pulse height zero in the log plot (see arrow) was obtained by
temporarily removing the input to
A
in
during data acquisition (situation as in Figure 3.25b left)
3b.7
The bCFD version of read-out of bipolar input signals
RoentDek
can equip all c versions of the standard CFD modules with modified
bCFD
boards on all or on selected
channels (mixed units).
bCFD
boards require bipolar input signals (with leading edge falling) as from
RoentDek
bFAMP
modules (see
). The
bCFD
circuit takes advantage of the fact that a bipolar signal already resembles the signal
shape as in Figure 3.4b. If an input signal has already the “right” shape it can be routed directly to the walk comparator and
additionally to the leading edge comparator (for setting the
CFD threshold
via the respective potentiometer).
