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The Constant Fraction Discriminators CFD8c, CFD7x, CFD4c, CFD1c and CFD1x (11.0.1701.1)
3b.2
Principles of the CFD circuit
It is important to note that an optimal operation of the
RoentDek
CFD units requires optimal settings of the parameters
•
Threshold level (Th)
•
Walk level (Z)
•
CFD delay
•
CFD fraction
with respect to the input signal properties. This will be explained in the following.
Figure 3.3b: Front panel inputs, test points and control potentiometers , not shown is the veto input
and the
CFD fraction
potentiometer being on the front panel for c and x versions (see Figure 3.14)
If a signal as shown in Figure 3.4b (left)
is turned into a bipolar signal (right) by an appropriate electronic chain it can be
shown that the zero-crossing of the bipolar signal does not jitter in time as function of the input pulse height.
The CFD circuit consists of two parts:
The “
analogue
”
circuit
produces this bipolar signal from a unipolar input signal: A resistor-divider splits the signal into two
with different relative pulse heights. The ratio of the pulse heights (
CFD fraction
) is determined by the choice of fix resistors
or a potentiometer (as in case of the
RoentDek
CFDs
). While one of the signals is inverted later on, the other experiences a
certain delay (
CFD delay
) before both signals are superimposed to form the bipolar signal. On the
RoentDek
CFDs
, this
delay is formed by an external coax cable (LEMO 00 series) connected between the front-panel sockets labelled
Delay
. The
bipolar signal can be observed on the analogue walk monitor output socket labelled M
a
(damped by a factor of 10). The
CFD
fraction
can be adjusted via a potentiometer on the front panel side panel (
CFD4b
: side panel,
CFD1b
and
CFD8b
: on the lid).
Figure 3.4b: Schematics of a CFD analogue chain
Figure 3.6b shows the digital circuit chain of two comparators and a logical AND-gate: One comparator switches to “high”
when the input signal exceeds a certain (adjustable) reference value (the
Threshold level)
and produces a norm signal (this is
how a simple “leading-edge” discriminator unit operates). When the input signal level is below the
Threshold level
, its output
returns to “low”. Ideally, the
Threshold level
is set above electronic noise but low enough to register even the smallest valid input
signals.
*
Note that for
RoentDek
CFDs
the input signal polarity is negative
