6
4.6. INPUT PROTECTION
A provision is built into the preamplifier to protect
the input FET stage from damage when high-
voltage transients are applied to its input. These
transients can result from any one or more of many
causes, including detector breakdown, moisture
condensation on the input connector, short circuits
or uncharged capacitance connected across the
input while bias is being applied through the
preamplifier, or disconnection of a bias voltage
without first reducing it gradually to zero.
The protection circuit is installed in the preamplifier
when the unit is shipped from the factory. Although
it offers protection to the FET, it also causes some
degradation of the noise performance of the
preamplifier, which increases as detector capacity
increases.
With the protection circuit in, the collector lead of
Q11 is attached to the center tap of the two series
input resistors to the FET, R39 and R5. Transistor
Q11 is connected as a diode, with both the base
and the emitter tied through R8 to ground. This
prevents the voltage in the input circuit from rising
beyond the safe limit for the FET input. To take the
protection circuit out, simply remove the collector
lead of Q11 from its circuit connection and install a
wire jumper across R39 and R5.
Inherent to all high-speed charge-sensitive
preamplifiers is the problem of keeping the total
system stable while interconnecting the detector
and preamplifier with long cables. To help ease this
problem and permit more flexibility for installations,
the protection circuit is designed to serve not only
as a protection for the FET input but also to
terminate the input cable in a nominal 100
S
. So it
is recommended that the protection circuit be left in
whenever possible.
In order to take full advantage of the risetime
capabilities of the 142PC for timing experiments
(typical risetimes of 25 to 150 ns for detector
capacities from 1 to 100 pF), the total cable length
should be kept as short as possible, even though it
is terminated. Due to vagaries in the system
installation
!
ground loops, stray inductances,
etc.
!
and since the maximum cable length is a
factor in the input capacitance to the preamplifier,
it is not possible to give absolute numbers.
Generally, two feet is a typical maximum length to
obtain fastest risetimes for low detector capacities.
Of course, the system can be compensated for
cable lengths of up to 5 or 6 feet but slower
risetimes will be obtained. The screwdriver
adjustment inside the case can be used to stabilize
the system and minimize the pulse risetime.
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 in
the following steps.
1. Furnish a voltage pulse to the Test Pulse
connector as outlined in Section 3.4. The polarity of
the test pulse signal should agree with the expected
signal input polarity from a proportional counter.
2. Using a calibrated pulser, the 142PC output,
either E or T, should be inverted from the input and
have a nominal scale factor of 300 mV per 1 MeV
equivalent energy (Si).
3. The noise contribution of the preamplifier may be
verified by two basic methods. In either case, the
Summary of Contents for 142PC
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