Advanced Measurement Technology 142AH Betriebs- und Wartungshandbuch - Seite 10
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A voltage test pulse for energy calibration can be
accepted through the Test input connector on the
142AH without the use of an external charge
terminator. The Test input of the preamplifiers
has an input impedance of 93
provides charge injection to the preamplifier
input. The shape of this pulse should be a fast
rise time (less than 40 ns) followed by a slow
exponential decay back to the baseline (200 to
400 µs). While test pulses are being furnished to
the Test input, connect either the detector (with
bias applied) or its equivalent capacitance to the
Input connector on the 142AH.
The Test input may be used in conjunction with a
pulser such as the ORTEC 419 or 448 to calibrate
the preamplifier E output amplitude in terms of
energy or for multichannel analyzer calibration.
However, due to stray coupling between the test
circuit and other portions of the preamplifier
circuitry, the transient performance of the
preamplifier is best determined by connecting the
actual detector signal through the Input connector
instead of using the pulse generator signals.
A voltage test pulse for transient response in the
142AH can be accepted through a charge
terminator and into the Detector Input connector.
If external capacitance is to be included for these
tests, an SHV tee can be inserted between the
Input connector and the charge terminator, and
this will then accommodate the test capacitances.
Do not furnish any bias during these tests.
3.6. DETECTOR BIAS INPUT
Operating bias for the detector is supplied to the
Bias connector on the 142AH and through a filter
and large bias resistance to the Input signal
connector. From there it is furnished out through
the signal input cable to the detector.
Connect a cable from the detector bias supply
(ORTEC 459 is typical) to the Bias connector on
the 142AH. The connectors used in this high-
voltage circuit are type SHV, and the mating
cable should be furnished with the bias supply
module
, and its circuitry
4
3.7. CORONA ELIMINATION
Because the normal range of bias voltages that
are used for the detectors appropriate to the
142AH are extremely high, dust particles that
settle within the Input connector tend to cause
spiking when the high voltage is applied. The
effect can be observed directly at the E output
connector on the 142AH with no input signal.
Figure 3.1 shows the typical output signal with
large noise spikes due to a "dirty" Input connector.
This can usually be eliminated, so that the output
has an appearance like Fig. 3.2, by disconnecting
the input cable (with the protection circuit
installed) and blowing out the inner surfaces of
the SHV connector with the "corona blower" that
is furnished with the 142AH. In particularly
stubborn cases, an orderly procedure must be
used to determine the cause of spiking; a
recommended procedure is as follows:
1. With no cable connected to the preamplifier
Input, operate the preamplifier at the desired bias
voltage while observing the E output, through a
shaping amplifier, with an oscilloscope.
2. If spiking is present, blow out the Input
connector with the blower several times until the
spiking disappears.
3. Attach the input cable from the detector. If
spiking appears, disconnect the cable and blow
out the inner surfaces of the high-voltage
connectors. Then attach the cable again and
check for spiking.
4. Apply this procedure to the connectors of each
individual cable section that has high-voltage
connectors.
5. In laboratory areas that are particularly dusty,
this procedure may have to be repeated several
times to completely eliminate all traces of spiking.
.