PUB96-32 Rev 2 August 29, 2005
32-3
TX Control Panel 40D1985G1
output
rises,
the
DC
voltage
increases,
and
this
increases
the
amount
of
attenuation,
thus
the
output
is
maintained
at
a
constant
level.
The
AGC
processing
is
done
by
analog
op
‐
amp
circuits
in
the
Metering
Board,
but
the
initial
threshold
setting
is
done
in
the
Control
board
from
AGC
switch
S5
and
AGC
potentiometer
R9.
These
simply
provide
an
adjustable
reference
bias
voltage
to
the
AGC
circuit,
which
adjusts
the
power
output
inversely
according
to
this
bias
voltage.
When
S5
is
open
(the
LED
in
S5
is
off
and
AGC
is
Disabled),
R9
rises
to
the
+12V
rail
of
the
Metering
Board
and
the
AGC
processing
stage
inverts
this
high
voltage
so
its
output
and
thus
the
AGC
voltage
is
very
low,
resulting
in
maximum
exciter
output.
Exciter
output
is
preadjusted
with
AGC
off,
to
make
110%
transmitter
power.
In
the
event
of
a
VSWR
that
exceeds
a
preset
amount,
the
AGC
voltage
becomes
modified
a
little
to
reduce
the
transmitter
output
by
an
amount
proportional
to
the
reflected
signal.
This
"VSWR
Cutback"
permits
the
transmitter
to
remain
on
the
air
at
reduced
power
if
the
antenna
should
gradually
accumulate
a
layer
of
ice.
If
the
reflected
power
should
exceed
a
much
larger
amount
causing
repeated
momentary
tripping
off
air
and
the
VSWR
supervisory
circuit
attempts
to
restore
transmitter
operation
but
cannot
and
then
locks
out,
then
the
AGC
voltage
is
cut
down
even
more.
This
is
called
"VSWR
Shutdown."
The
AGC
voltage
and
modifications
to
it
from
VSWR,
are
summed
in
U2A
which
is
basically
a
buffer
amplifier
that
also
provides
a
telemetry
output
to
the
remote
control
system
through
J5
‐
3.
R15
sets
the
calibration
of
the
meter
when
it
reads
from
the
AGC
position
of
the
meter
selector
switch
S1.
R1
sets
the
calibration
of
the
meter
when
it
reads
the
IPA
output
level
from
the
IPA
position
of
S1.
Forward
and
Reflected
meter
calibration
is
done
with
potentiometers
on
the
Metering
Board.
