WMBJ GSM.5OO
3.0THEORY OF OPERATION
3.1 GENERAL:
The GSM-500 utilizes
the latest
field proven,
microprocessor
circuitry and internal probe response
curvesto operate
an internal energy
compensatedGM detector and a variety of detector from the external input. The instrument can
function as
a regular ratemeter
without deadtime compensation,regular ratemeter
with dead
time compensationand
integrating meter. Range
changing is automatic for all ranges
and both internal and external inputs have separatenon
interacting controls for dosecalibration. Calibration of the counts/minute
scales
for both detectors
is AUTOMATIC
and not required in the field. Operation in either the counts/minute
or dose
modeis
switch selectable
from the front
panel. The electronic
circuitry is located
on 2 heavy
duty printed circuit boards and is
powered
by highly regulated
-5
vdc & +5 VDC (+ 0.5%) low temperature coefficient
power supplies. A separatehigh stability power supply
with a low
temperature coefficient
can be adjusted
to provide a different
operating voltage fioreachdetector
(300
- 1500vdc
(ll%). Diagnostic
circuitry constantlymonitors the power supply, high voltage
supply and GM detector
(saturation)
and indicates
out of tolerance
operation with RED LED's and by locking
the meter full scale
for saturation. An analog
meter
with a large 4.22" scale
indicates
the probe signal level
and operating range.The probes
are connectedto the
main electronics
housing
by a high quality coaxial cablewith quick disconnectBNC connectors.
3.2 DETECTOR SIGNAL - DIAGNOSTICS - CALIBRATION CONTROLS
A microprocessor
meter operatesmuch
differently than an analog
type meter.
Most of the functions i.e.
deadtime
correction,
diagnostics
etc.
are provided by the microprocessor
programming (software).
The GSM-500's
hasbeen
pre-
programmed with the response
curves
and deadtime correction for 6 different probes.
Switch SW4 is
the internal
PROBE SELECTION switch that must
be ADJUSTED TO THE CORRECT PROBE BEFORE CALIBRATION. The
position
of SW4 (0-5)
selects
the correct software
for the probe being utilized on the internal and external probe.
SW3
tellsthe microprocessor
which detector
is being calibrated and SW5 serves
severalfunctions i.e.acknowledgecal pots
have
beenset
correctly and to showalarm and timed count set
point on the panel
meter. In this system
the calibration
potimeters
Pl -P4 are utilized to calibrate both the #1 and
#2 input. The software
selectedfor eachdetector
(internal
and external) includes
the DEAD TIME CORRECTION for the CPM operating mode,the High Voltage setting for
the probe selected
and RESPONSE
LINEARIZATION for the probe in the DOSE mode.THE INSTRUMENT MUST
BE OPERATED IN THE *O'ZERO POSITION AFTER THE CALIBRATIONS HAVE BEEN COMPLETED. When
the calibration ofeach probe has
beencompleted
the operator only needs
to select
the desired
probe (internal or
external)
and all settings
and adjustmentsare automatically applied to the probe
selected.
The GSM-500 needs
only to
be
calibrated in the DOSE MODE (referenced
csl37)for probes
connected
to either input. The internal probe selection
switch must
be adjusted
to the calibration position
and correct probe position
and the range
potentiometers
P1-P4
AdJusted.
THIS PROCEDURE MUST BE COMPLETED FOR EACH INPUT FOR DOSE CALIBRATION ONLY.
NO CALIBRATION IS REQUIRED FOR ANY PROBE IN THE CpM MODE. THE GSM-500 IS
AUTOMATICALLY CALIBRATED IN THE CPM MODE BY THE MICROPROCESSOR. A list of the probes
and
corresponding
switch positions
are on a label in the bottom housing
and in section6.2.5
of this manual The digital
signal from the internal and external
probe is connected
to through connector
UW-100 and C12 to pin 2 of U6
depending
on the setting
of the MAIN SELECTOR SWITCH. The MAIN SELECTOR SWITCH selectsthe probe
INPUT SIGNAL internal or external. Figure I showsthe front panel
controls
of the GSM-500.
3.3
SIGNAL CONDITIONING
The INTERITIAL and EXTERNAL probe
signal
are routed to the main circuitry by the MAIN SELECTOR SWITCH
SW3. The signal from the probe
selected
is conditioned
and amplified though part of U6 and then connectedto pin 4 of
U7.U7 is
a comparator that functions
asa minimum signal amplitude that can
be
processed.
This stage
is utilized to
discriminate against low-level
"noise" signalsand low energy radiation pulses
that could interfere with the radiation
signals
of interest.
The minimum probe
signatthat can be
processed
at MAXIMUM sensitivity
is 0.050
V-pp. All
signals
exceeding
5 V-pp are limited to protect the internal circuitry. The signal from U7 is
utilized for 2 separate
functions
- an audible click for eachinput pulse
and the digitat signal
(equilivant to the probe signat)to drive the
microprocessor.
The signal
from U7 is connected
to pin 12
of U5 - a oneshot multivibrator that generates
the audible
click for each
pulse.
The signal
from U7 is
alsoconnected
to pin 27
(input PA7) of Ul the microprocessorthat
completes
all of the signal conditioning,
scaling,
linearizing etc.
CoPYRIGHT@
2006
WMB.
JOHNSON
& ASSOCTATES
