5
4. OPERATION
4.1. GENERAL
Figure 4.1 is a simplified block diagram of the
circuits in the 142AH Preamplifier. When the
protection circuit is in, the diode between the two
series input resistors to the amplifier stage is
connected. When the protection circuit is out, the
diode is disconnected and a jumper is used to
short across the two series input resistors.
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 the
load resistance (approximately 100 megohms) to
the Input SHV 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 level is adjusted.
When the detector bias must be raised to a level
greater than 3000V, there are inherent problems
associated with obtaining low noise and high
performance. Careful design and good
manufacturing control, as well as extensive
testing under full bias, assure that each
preamplifier is free from spiking problems and
that no degradation of noise performance will
result when the required bias voltage is applied.
But dust, dirt, and other surface contamination
can collect within the Input connector and cause
spiking problems due to corona discharge in the
Input connector and associated cabling. This is
especially true when the contamination occurs on
the voltage-carrying center pin of the Input
connector while the preamplifier is operating with
bias voltages greater than 3000 V. These
problems can usually be cured by directing a
stream of clean compressed air or nitrogen into
the end of the connector to blow off the surface
contamination. This is a very subtle problem
source because enough contamination can
reaccumulate on the Input connector within
several seconds after cleaning and cause
recurrence of the corona. A dust cap is provided
to keep the Input connector covered when it is not
in use, and this will protect the interior surfaces
when a cable is not connected. When the cable is
connected, it can also transfer contaminants to
the connector that can cause some spiking
problems, even after the cable is removed.
