5
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
142 can be accepted through a charge terminator
and into the detector Input connector. If external
capacitance is to be included for these tests, a BNC
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 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 428 is typical) to the Bias connector on the
142. The connectors used in this high-voltage
circuit are type SHV.
4. OPERATION
4.1. GENERAL
Figure 4.1 is a simplified block diagram of the
circuitry in the 142A, 142B, and 142C Preamplifiers.
Capacitor C34 is not included in the 142A but is
included in the 142B and C. Resistor R3 is furnished
as an accessory to the preamplifier and can be
installed in parallel with R4 for those applications
where the detector leakage current is great enough
to cause too much voltage drop across R4. Where
the leakage current is small, R3 should not be
installed in the circuit because it would tend to
degrade the noise performance.
4.2. DETECTOR BIAS
The amount of bias required by the detector is
specified in the data furnished with the detector.
The bias accepted into the preamplifier through the
SHV Bias connector is furnished through R2 and R4
(approximately 100 M
S
) to the Input BNC
connector of the preamplifier. If the detector
leakage current is appreciable, a notable voltage
drop will occur across the series load resistor in the
preampliflier, and this must be added to the detector
requirement when the bias supply is adjusted.
When a high-leakage detector is to be used and its
drop across the load resistor would be excessive,
the load resistance can be decreased by installing
R3, the 10-M
S
resistor, in parallel with R4 as
indicated in Fig. 4.1. This must be done very
carefully to prevent damage to the preamplifier
circuits and requires that the case be opened. Use
the following suggestions:
1. Remove the source of bias and short the Bias
connector for at least 20 seconds (see Notice on
page iv).
2. Remove the case to expose the preamplifier
circuits. Locate but do not touch the 100-M
S
resistor, R4. Resistor R3 will be soldered in parallel
with R4.
3. Be very careful to prevent excessive heating of
any components in the preamplifier while soldering
the leads of R3 to the leads of R4 so that the
resistances are in parallel. Be sure that each solder
joint is clean, smooth, and shiny after assembly;
when high voltage is applied to this circuit, corona
leakage may develop at any sharp points that may
be left.
4. When operation with a low-leakage detector is
desired, remove R3 from the circuit carefully,
observing all of the above precautions.
