The Constant Fraction Discriminators CFD8c, CFD7x, CFD4c, CFD1c and CFD1x (11.0.1701.1)
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: Rule for Delay
Figure 3.8b: Signal traces similar as in Figure 3.7b for poorly adjusted
Threshold level:
analogue monitor output,
digital monitor output (only in left picture) and CFD output. Left picture: for too high threshold settings only input
signals with very high amplitudes are registered. When setting the correct Threshold level, the dark area shown by
the arrow would be filled with smaller amplitude signals. Right picture: threshold setting too low (i.e. in the noise)
Sometimes it is necessary to compromise and allow a certain amount of noise triggers in situations where the noise level is not
well separated from the signals. This is acceptable if the random signals do not affect the measurement and their rate does not
exhaust the data acquisition capability of circuits in follow-up electronics (e.g. a TDC or TAC). If the threshold is set too high,
signals are lost.
3b.3.2
CFD delay
The
CFD delay
must correspond to the input signal rise time and is set by bridging the “Delay”-sockets with an appropriate
cable (LEMO 00 series). As a thumb rule, a 1m long coax cable will produce an external delay of 5 ns. It is to note that the
RoentDek
CFDs have an additional internal offset delay of about 0.5 ns. A 1 m (5 ns) external delay cable thus results in a
total delay D of 5.5 ns. Note, that the
RoentDek
CFDs will not operate without connecting an external delay cable bridge.
If the pulse rise time RT is defined as the time from reaching 10% to 90% of the signal maximum, the delay shall be equal or
smaller (50-80%) than this rise time RT, depending on the
CFD fraction
ratio f. For small
CFD fractions
(< 0.5) a thumb rule for
an appropriate delay D is
D = RT (1-f)
However, a delay D < ½ RT is not recommended even for CFD fractions higher than 0.5.
If a
RoentDek
CFD is delivered as part of a detector system, appropriate cables are usually included. Cables with 1-3 ns
delay shall be used for the MCP timing signal and cables with 4 or 5 ns delay are recommended for the delay-line signals (10 ns
in case of LC-delay-line anodes). Sometimes, especially for large-size detectors it may be of advantage to use longer delay
cables.
The input pulses in Figure 3.7b have about 6 ns rise time, the chosen cable delay results in a CFD delay of 4.5 ns, which is
close enough to the optimal value of 4 ns for the selected CFD fraction of 0.35. If the delay cable is too long, the two
intermediate signals (inverted/delayed, respectively) do not merge into a “proper” bipolar signal (see Figure 3.9b left). If the
delay cable is too short, no distinct cross over is formed from the intermediate signals (see Figure 3.9b right). The CFD cannot
function properly under any such
CFD delay
settings. A non-perfect choice of delay can sometimes be compensated by
adjusting the
CFD fraction
(see below) as long as D/RT is between 0.5 and 1 (see Figure 3.10b).
Equation 3.1b
