8
preamplifier, which increases as detector capacity
increases.
With the protection circuit in, the emitter lead of
Q11 is attached to the input of the first FET stage
and this prevents the voltage at that point from
increasing beyond the safe range for the FET input.
Resistor R5 protects both the clamp and the FET
from damage. To take the protection circuit out,
simply remove the emitter lead of Q11 from its
circuit connection and install a wire jumper across
R5. A formed wire jumper is included as an
accessory in the shipping bag and is to be used for
this purpose when operation is desired with the
protection circuit bypassed.
5. MAINTENANCE INSTRUCTIONS
5.1. TESTING PERFORMANCE
As ordinarily used in a counting or spectroscopy
system, the preamplifier is one part of a series
system involving the source of particles to be
analyzed, the detector, the preamplifier, the main
amplifier, and the pulse height analyzer. When
proper results are not being obtained and tests for
proper performance of the preamplifier and the
other components are indicated, it is important to
realize that rapid and logical testing is possible only
when the individual components are separated from
the system. In proving the performance of the
preamplifier, it should be removed from the system
and be dealt with alone, by providing a known
electrical input signal and testing for the proper
output signals with an oscilloscope as specified
below.
1. Furnish a voltage pulse to the Test connector, as
outlined in Section 3.5. The polarity of the test pulse
signal should agree with the expected signal input
polarity from a detector.
2. Using a calibrated pulser, the 142A E output
should be inverted from the input polarity and
should have a nominal scale factor of 45 mV output
per 1 MeV equivalent energy (Si). The 142B and C
E outputs should also be inverted from the input
polarity and have about 20 mV per 1 MeV input
equivalent energy. The timing outputs should have
the same polarity as the inputs with a scale factor of
about 20% less than the signals through the E
outputs.
3. The noise contribution of the preamplifier may be
verified by two basic methods. In either case, the
normal capacity of the detector and associated
cables should be replaced by a capacitor of equal
value connected to the Input connector. This is
necessary because the noise contribution of the
preamplifier is dependent upon input capacity, as
can be seen from the noise specifications given in
Section 2. The only meaningful statement of the
noise level of the preamplifier is one that relates to
the spread caused by the noise in actual spectra.
This can be measured and expressed in terms of
the full width at half maximum (FWHM) of a
monoenergetic signal after passing through the
preamplifier and main amplifier system. The noise
performance referenced in Section 2 is stated in
these terms, and verification methods will be
described. If desired, the preamplifier can be tested
with no external capacity on the Input connector, in
which case the noise width should be approximately
that shown for zero external capacity. In any case,
the input connector and capacitors, when used,
should be completely shielded electrically. A
wrapping of aluminum foil around the Input
connector or a shielding cap attached to the
connector will suffice for testing at zero capacity.
4. The preamplifier must be tested in conjunction
with an associated main amplifier that provides the
required pulse shaping. The typical noise
performance given in Section 2 is obtained using an
ORTEC 572 Spectroscopy Amplifier on which the
time constants have been set as specified. For
comparison of these tabulated values, it is
preferable to test the preamplifier under identical
pulse-shaping conditions. It is also important to
ensure that the noise level of the input stage of the
associated main amplifier does not contribute
materially to the total noise. This is usually no
problem provided that input attenuators, if any, on
the main amplifier are set for minimum attenuation.
