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The Constant Fraction Discriminators CFD8c, CFD7x, CFD4c, CFD1c and CFD1x (11.0.1701.1)
Figure 3.7b: Oscilloscope traces of in- and output signals of the CFD, traces 1-4 from top: input signal, analogue
monitor output signal, digital monitor output signal, CFD timing output signal (which triggers all traces). The red
arrows indicate the
timing transition
which appears slightly shifted in the different traces due to internal delays.
This and the following images of signal traces have been obtained with a 300 MHz analogue oscilloscope (a large
number of signals with varying pulse heights are superimposed). Similar views can be obtained from digital
oscilloscopes with adequate “persistence” setting
The
Threshold level
(see below) was set so that even the smaller signals from the pulse height distribution are registered. This can
be verified both on the first trace (input signal) and on the analogue monitor output (trace 2). The input signal cannot always
be displayed together with the CFD outputs unless the signal source (e.g. amplifier) has two independent outputs sockets.
Therefore the trace of the signal input was omitted on the next figures. The analogue walk monitor may serve as well for
verifying the threshold level but it is to note that the analogue walk monitor signal height is damped by a factor of 10.
For achieving the ideal settings it is first of all important to note that the CFD operates best (in all respects) and yields the
optimal temporal resolution if the input signals are as high as possible but only up to -2 V. The threshold should not be set
smaller than 50 mV for the multi-channel versions because then there is a risk of inter-channel cross talk (see Chapter 3b.5.3).
Please note that it is preferable to obtain such high signals rather by a low amplifier gain but high “raw” signal from the given
detector (e.g. high gain on the physical electron multiplier MCP, PMT, Channeltron …) if one has this choice. This can for
example be achieved by increasing the bias voltage on the physical electron multiplier.
For a given electronic noise level, signal-to-noise ratio can only be improved by increasing the gain of the primary charge input
from the detector source to the electronics. Only then signals have optimal signal-to-noise ratio which will finally determine
achievable resolution.
3b.3.1
Threshold level
The
Threshold level
shall be set just above the input signal line’s noise level. This can be verified by observing the CFD output
simultaneously with the analogue walk output on an oscilloscope as shown in Figure 3.8b.
If the threshold is set too high, small (but valid) signals are not registered (see Figure 3.8b, left picture), if it is set too low (right
picture), the CFD output will also trigger on spurious noise components and produce background counts.
