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The Constant Fraction Discriminators CFD8c, CFD7x, CFD4c, CFD1c and CFD1x (11.0.1701.1)
Figure 3.23b: CFD1x (left) and close-up of the CFD7x (right) showing the additional
input/output sockets, switch and control potentiometers for the
CFDx
circuit
In the
RoentDek
CFDs
with
CFDx
function the respective channel consists of two parts: the standard CFD part is
identical to other CFD channels (there is only one
CFD Out
socket less). In order to allow for a proper pulse height
measurement the analogue input signal is (after shaping and inverting to positive polarity) routed to the additional
A
out
socket.
From there it has to be connected to the
A
in
socket and thus enters the
CFDx
part of the circuit where it can be verified on
the
ramp
monitor output socket labelled
M
Ramp
(see Figure 3.24b). The reason of this external routing is that accurate
measurement of peak pulse height requires choosing the proper cable length (which is very close to the cable length as used
for the
CFD delay
). This will be explained below.
Figure 3.24b: Signal traces of input signals. Upper trace: as supplied to the CFD’s
In
socket
(obtained from an alternative amplifier output), lower trace:
ramp monitor
output (the
Ramp
switch is turned off here). Signals are triggered on the CFD timing output signal (trace not shown).
Left and right pictures were obtained with two different cable lengths between
A
in
and
A
out
sockets
For coding input signal pulse height into a time delay the input signal (entered through the
A
in
socket) is superimposed with an
internal so-called
ramp
signal. At a fixed time after the leading edges of the CFD timing (and CFDx) output signals the
triangular-shaped
ramp
signal (see Figure 3.25b left) is internally formed by a level shift as function of time with almost
constant negative slope until a certain value is reached (between -0.5 V and -1 V as set via the potentiometer labelled
Width
Offset
). A comparator chip (“
ramp comparator
”) is then triggered and re-sets the ramp level to 0 V. At the same time it triggers
the trailing edge of the
CFDx
output signal and additionally initiates (the leading edge of) a NIM signal on the
Stop
socket
(accordingly labelled). The length of this
stop
signal can be adjusted via the potentiometer labelled
Stop
width
.
*
In principle, any signal can be entered here. The signal/level supplied to
A
in
must be of positive polarity and not exceed 2V at
any time. Please contact
RoentDek
for
CFD1x
version with the option of accordingly coding an external signal’s pulse
height or a DC level into a delay.
