The Constant Fraction Discriminators CFD8c, CFD7x, CFD4c, CFD1c and CFD1x (11.0.1701.1)
Page 9 of 25
A second so-called “walk comparator” in the circuit operates in the
same way with the bipolar signal. Its reference, the
Walk level
, shall
be set very near and slightly above the baseline level of the input
signal so that it “triggers in the noise”. Then the bipolar signal
produces a distinct series of transitions as the noise fluctuates around
this level (see Figure 3.6b). In presence of a significant input signal a
certain transition from “low” to “high” represents the zero-crossing
of the bipolar signal (the “
timing transition
”) which shall serve as the
accurate timing output of the CFD circuit. This signal is produced by
a logic AND-gate of the two comparator outputs.
The
RoentDek
CFDs
allow monitoring the output of the walk
comparator via the digital monitor output socket labelled
M
d
.
The
Threshold level
and the
Walk level
can be set via two potentiometers labelled
Th
and
Z
(Z standing for Zero Crossing Level)
with corresponding test points to measure the actual levels with a voltmeter. A ground reference point labelled GND is
provided on the front panel of most units. The width of the CFD timing output signals is adjustable via a potentiometer
labelled
W
on the front panel. The width can be increased by a factor of 10 by placing a jumper on the dedicated position
through a hole in the side panel (
CFD8/7x
: top lid) for each channel individually.
Note that the test points can only give coarse information about the setting (within about ±5 mV). For optimal results and
adjustment, the signals have to be verified on an oscilloscope. The
CFD Fraction
can be adjusted by another potentiometer
labelled
Fr
on the front panel (
CFD4b
,
CFD1b
: side panel,
CFD8b
: top lid (see Figure 3.6b). Figure 3.6b also shows the
position of the jumper terminal for increasing the timing signal output width
The
RoentDek
CFD is useful for signals with rise/fall time > 1 ns. If the input signal source has less than 3 ns FWHM the
CFD might not operate optimally even with small CFD delay and optimized settings of the fraction (see below). The input
signal heights should be kept between 50 mV and -2 V (on 50
Ω impedance) by selecting a proper gain on the gi
ven amplifier.
The input should not be exposed to signals higher than -3 V or DC levels higher than ±1 V.
The oscilloscope traces in the next chapter are shown as examples for optimal parameter settings and the effect of changing a
certain parameter.
3b.3
Adjusting the CFD parameter
Figure 3.7b shows all relevant signals with a near-optimal adjustment of the parameters
Threshold level
,
Walk level
,
CFD delay
and
CFD fraction
for a given input signal of about 5 ns rise time (upper trace). The traces trigger on the negative slope of the lowest
trace, which is the CFD timing output. The analogue and digital walk monitor outputs are displayed on the second and third
trace. The
CFD fraction
was set to 0.35 (default) and cable produces 4 ns external delay which results in an effective
CFD delay
of about 4.5 ns, i.e. sightly increased by internal delays on the circuit board).
*
Note the M
d
output has a DC offset of about -0.1 V
Figure 3.6b: Schematics of the
CFD digital circuit chain
Figure 3.5b: top/side panel with fraction
potentiometer only versions a/b) and optional
jumper terminal for increasing the CFD output
width by a factor of 10 (optional)
