VHF OUTPUT RF METERING & AGC CIRCUIT BOARD
PUB96-33 rev 1: May 16, 1997
33-2
RF Output Metering 20B1299G3
is
output.
d
ve.
The
output
of
U1
‐
7
(U2
‐
7)
drives
the
RF
power
meter
through
R32
(R30)
which
set
the
meter
deflection
with
a
known
RF
signal.
U1
‐
7
(U2
‐
7)
drives
the
telemetry
buffer
U4
through
R29
(R47)
which
are
adjusted
to
calibrate
the
telemetry
to
a
standard
voltage
with
a
known
RF
signal.
Forward
calibration
is
done
with
full
rated
power
and
a
forward
RF
sample
from
the
probe
section
applied
to
J1.
R29
adjusted
for
3.0
VDC
delivered
to
J3
‐
6,
and
R32
is
adjusted
for
a
100%
reading
on
the
forward
power
meter
position.
For
Reflected
calibration,
the
same
forward
RF
sample
is
then
applied
through
a
16
dB
pad
to
J2,
and
R47
is
adjusted
for
1.5
VDC
at
J3
‐
10,
and
R30
is
set
for
a
2.5%
reading
on
the
Reflected
Power
meter.
Because
of
the
extra
16
dB,
the
calibration
automatically
gets
a
x40
multiplier,
so
the
actual
reading
of
the
meter
is
2.5%
on
a
full
scale
of
12.5%.
The
first
scale
mark
of
the
meter
is
then
0.5%.
If
you
were
to
substitute
a
10
dB
pad
for
the
16
dB
and
adjust
R36
and
R49
to
get
the
VSWR
supervision
levels
out
of
the
way,
you
would
see
a
meter
deflection
of
100%
which
corresponds
to
actual
10%
reflected
power,
and
a
telemetry
output
voltage
of
3.0
VDC.
It
is
simply
the
insertion
of
the
pad
and
the
subsequent
calibration,
that
provides
the
meter
multiplier
scale
factor.
When
you
are
setting
up
the
adjustments
on
this
board,
disable
the
AGC,
and
set
the
exciter
level
for
100%
RF
output
from
the
transmitter.
This
is
especially
true
for
the
reflected
or
VSWR
settings,
because
these
are
done
at
a
forced
RF
level
that
is
high
enough
that
the
AGC
system
will
be
driven
into
cutback
mode
which
is
what
will
need
to
be
set
up
because
it
affects
the
exciter
AGC
and
VSWR
supervision:
The
output
from
U1
(U2)
pin
7
is
also
applied
to
a
second
op
‐
amp
U1
(U2)
pin
3.
If
the
detected
level
at
U1
pin
3
rises
above
the
level
set
by
the
AGC
pot
(on
the
Control
board)
at
U1
pin
2,
the
output
on
pin
1
will
rise.
This
AGC
output
is
applied
via
J5
‐
1
to
a
final
buffer
amplifier
(U2A
on
the
Control
board)
an
from
there
to
a
PIN
attenuator
in
the
exciter,
thereby
reducing
the
transmitter
power
accordingly.
The
reflected
power
detector
CR2,
Q2,
CR4,
U2
is
similar
in
operation
to
the
forward
power
detector
CR1,
Q1,
CR3,
U1.
If
the
reflected
power
rises
to
a
value
higher
than
a
calibrated
value
of
17
to
18
dB
below
the
forward
power
level,
then
pin
3
of
U2
rises
above
the
voltage
on
pin
2
that
is
set
by
the
adjustment
of
R36,
U2
pin
1
will
go
higher,
which
applies
a
drive
cutback
signal
via
J5
‐
2
to
U2A
in
the
Control
board
and
from
there
to
the
PIN
attenuator
in
the
exciter.
At
the
same
time,
U3
pins
3
and
6
are
also
driven
more
positi
When
the
voltage
on
U3
‐
3
exceeds
the
voltage
on
U3
‐
2
that
is
determined
by
trip
threshold
control
R42,
U3
‐
1
goes
HIGH,
to
output
a
status
signal
HIGH
through
buffer
Q4,
and
to
energize
VSWR
trip
relay
K1
thru
buffer
Q3.
The
base
of
Q3
in
some
transmitters
may
have
a
time
delay
R61,
C21
added,
to
avoid
false
VSWR
tripping
after
power
failure
and
restoration.
The
component
parts
for
this
"fix"
may
be
soldered
to
the
back
of
the
board
in
Rev
4
and
earlier.
If
the
reflected
power
increases
still
further,
the
voltage
on
U3
‐
6
rises
past
the
voltage
threshold
set
by
R49
on
U3
‐
5,
then
U3
‐
7
will
switch
LOW,
which
triggers
the
"555"
timer
U5.
This
IC
will
produce
an
output
pulse
at
U5
pin
3
of
approximately
one
second
duration.
This
pulse
is
also
applied
to
the
PIN
attenuator
to
temporarily
remove
RF
drive.
Restoration
of
the
transmitter
should
normally
happen
