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b.
The
RF
signal
from
Q 1
is
fed
to
dual conversion receiver module U 1 which consists
of
a 1st
mixer
, 1st local oscillator, a
2nd
mixer
, a
2nd
local oscillator, limiting IF amplifier and an
FM
detector.
c.
A 10.7
MHz
1st IF frequency
is
generated
by
down
converting the incoming RF frequency
with
the
1st
LO
frequency.
The
10. 7
MHz
signal is filtered
at
FL
1
and
returned to U 1 for a
s
econd
down
conversion
to
450
kHz
.
d .
The
450
kHz
IF
signal is amplified
by
a built-in
IF
limiting amplifier and s
ent
to
an
FM
detector.
e.
The
detected audio signal is routed to relay K 1, along with the AUDIO
IN
from the backup
receiver module.
If
the module loses
RF
or
modulation,
and
the
AUTO
SWITCH
strap (not
shown)
is in the
ON
po
sition, the backup audio will
be
amplified
by
U6
for
output.
If
the
AUTO
SWITCH
strap
is
in
the
OFF
position,
the
receiver audio will routed
to
U6
for output.
Th
e audio output from the
FM
detector can be adju
st
ed
via potentiometer
R30
which is
accessible from
the
rear panel.
f.
Frequency selection is via a single-digit
BCD
rotary
DIP
switch along with microprocessor
U4
and
a
PLL
frequency sy
nthe
sizer consisting
of
U2,
U3, Q3
and
other
compone
nt
s.
g .
The
output at pin
10
of
U 1
is
a
DC
voltage analog
of
the
RSSI
(RF level)
of
the input signal.
Thi
s
output
is buffered
by
U7
A, and
can
be
measured with a
OC
voltmeter
at
TPl
which
is
accessible
by
removing the associated front panel
cover
plate.
-·
S17f
CHAMEL
NO
U4
l'\.1
,o:
n•"
us
MCM
5f70P
uP PIC1eC55HS
IO.HW<&
CLOCK
AUDIO
O\IT
,.,,,
~----
°""""
">-~L-1---.--~4
...,.;:. ' "
4
}-
ALNW
<Nf
Figure 5-4. NOAA
Weather
Receiver Module - Block Diagram
5-5
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