Operation
4
Operation
4.1 General
The purpose of this section is to familiarize the user with the operation of the model 2128N and to
prove that the unit is working properly. The module can be operated in many different system
configurations, therefore it is difficult to give explicit operating instructions. However, if the guide line
of the listed procedures is followed, the experimentalist will gain sufficient experience with this unit in
order to use it properly.
4.2 Laboratory bench tests
Basic performance tests of the model 2128N may be exercised either in a rack mounted NIM-BIN
(
N
uclear
I
nstrumentation
M
odule) power supply or on a laboratory bench with the unit powered by an
extender cable from a NIM-BIN (TID-20893). It is recommended that electrical connections be made
with BIN power off.
4.2.1 Input
Suitable driving pulses may be obtained from laboratory pulsers or pulse generators, or readily
available detector pulse signals. The input network is protected for pulses exceeding 5V in
amplitude. But the user should keep in mind that the linearity is limited by the common mode range
of the ultra fast comparators in the input stages.
4.2.2 Operating mode
The selection of the operating mode depends on the requirements of the experiment.
In the
CFT
mode (normal constant fraction) the timing is derived from a comparison between an
attenuated pulse (fraction f = 0.4; factory set; f can be easily changed by the user) and a delayed
pulse (delay time t
d
; external delay cable). The derived time mark is (theoretically) independent of the
pulse height of the input signal. For a detailed description of the principle see e.g.:
D.A. Gedcke and W.J. McDonald, Nucl. Instr.& Meth. 56(1968)253
M.R. Maier and P. Sperr, Nucl. Instr. & Meth. 87(1970)13
The choice of the fraction f and the time t
d
(see also section 4.2.3) depends on the detector and the
experimental setup.
In the
CFRR
mode (constant fraction with slow rise time reject) an option is provided to handle
longer rise times (e.g. solid state Ge-detectors). Normally the timing mark for the CF is derived with
the presumption that the threshold level has been set quite low and this level is exceeded prior to to
the derivation of the time mark. In the case of long varying wavefronts of the input pulses (and if
relatively short delay cables are used) the timing conditions as given above may not be valid. In that
case the level discriminator may switch late. The resulting timing mark will represent some mixture of
the intended CFT timing with leading edge timing. This effect causes tails or even a satellite peak in
a time spectrum. In the CFRR mode an option is given which rejects pulses which have not
exceeded the threshold level prior to the derived CF timing mark. Some loss in the count rate is
obvious.
In the
LET
mode the model 2128N can be used as a simple leading edge trigger up to very high
count rates. The leading edge mode should only be used with signals having a very short rise time
and a very limited range of amplitudes as change in amplitude will cause walk (see also section 5.1).
10
F
ComTec GmbH
