Icom IC-R8500 Service Manual page 11

(1)
WFM
mode
The
filtered
10.7
MHz
2nd
IF signal
from the
bandpass
filter
(FI2)
is
applied
to
the
WFM
demodulator
circuit ()C2, pin
7).
The
applied IF signal
is
mixed
with
an
LO
signal
which
Is
generated by XI
to
demodulate
AF
signals.
The demodu-
lated
AF
signals
are output from
pin
13,
and are then
applied
to
the squelch
control
gate
(ICl9d).
(2)
FM mode
The
amplified 3rd IF signal at the
IF amplifier (Q16)
is
again
amplified
at
the other
IF amplifier (Q64)
and then applied
to
the
FM
detector
circuit (ICS, pin
5).
The
applied 3rd IF
signal
is
mixed
with the signal
generated by
discriminator
(X2)
to
demodulates
AF
signals.
The demodulated
AF
signals
are output from
pin 9
and passed
through
the active
filter
circuit
(IC32a)
to
suppress
unwanted
subaudible tone
audio
signals
and lower noise components.
The
filtered
AF
signals are applied
to
the squelch
control
gate
(ICl9c).
(3)
AM
mode
The
amplified 3rd IF signal
from
the
IF amplifier (Q16)
is
again amplified
at
the other
IF
amplifiers (Ql7, Q18).
The
amplified 3rd
IF signal
is
then
applied
to
the
AM
demodulator
circuit (015).
The
demodulated
AF
signals
are
applied
to
the squelch
control gate (IC19b).
(4)
S5B
and
CW
modes
The
amplified 3rd
IF signal
from the
IF amplifier
(Q18)
is
applied to the
balanced mixer
circuit (IC7)
and mixed
with
BFO
signals
to
demodulate
AF
signals.
The
demodulated
AF
signals are applied
to the
squelch
control
gate
(ICl9a).
The
BFO
signals are generated
at
the
BFO
circuit
(PLL
unit,
IC15)
and are applied
to
the
balanced mixer
circuit
via the
buffer-amplifier
(Q31).
4-1-13
SQUELCH CONTROL
CIRCUIT
(MAIN
UNIT)
The demodulated
AF
signals from
the
demodulator
circuits
are applied
to
the squelch
control
gate
(IC19).
This con-
sists
of
4 analog switches which are
selected with a
mode
signal
2
und squelch
control setting
from
the
CPU
(IC33) via
the
expander
1C (IC18).
The
switched
AF
signals are
applied
to
the
AF
circuit.
4-
1-14
SQUELCH
CIRCUIT (MAIN
UNIT)
(1)
FM. FM-N,
AM
and
AM-W
modes
A
squelch
circuit
cuts out
AF
signals
when
no
RF
signal
is
received
or the
S-meter
signal
is
lower than the
[SQUELCH]
control setting
level.
By
detecting noise
components
in
the
AF
signals,
the
CPU
switches the squelch
control gate.
Some
noise
components
in
the
AF
signals from
pin
9
of the
FM
IF
iC (ICS) are applied
to
the noise
filter
section
in
the
FM
IF IC
(pin 10).
The
filtered
noise
components
are output
from
pin
11,
and
then applied
to
the noise
amplifier
circuit
(IC9b).
The
amplified signals are
rectified at
the noise
detector
circuit
(D16)
and
the detected voltages are applied
to
the
CPU
(IC33) via
the
NOAD
line after
being
current-
amplified
at
the current-amplifier
circuit (IC4a).
The [SQUELCH]
control level signal
is
applied
to
the
CPU
(IC33) via the
sub-CPU
(FRONT
unit,
IC1)
as a
reference
voltage
for
comparison
with the noise voltages.
Also,
an
5-
meter
signal
is
applied
to
the
CPU
from
the meter
ampli-
fier
(IC9a).
The CPU
compares
these
signals, then outputs
a
control signal to
the
squelch
control gate.
The
FM
IF
IC detects noise
components even
in
AM
and
AM-W
modes
for
noise squelch
control,
and
also the IF
amplifier (Q18)
is
activated
in
FM
and
FM-N modes
for
S-meter
and
S-meter squelch
functions.
(2)
WFM,
S5B,
CW
and
AM-N modes
The
squelch
circuit
mutes
AF
output
when
the S-meJter
signal
is
lower than the
[SQUELCH]
control setting
level.
*
SSB,
CW
and
AM-N modes
A
portion of the 3rd IF signal
from
the IF amplifier
(Q18)
Is
converted
into
DC
voltage
at
the
AGC
detector
(Q27.
D20)
and
the
meter
amplifier (lC9a).
The
amplified signal
is
applied
to
the
CPU
(IC33) via the
SMAD
line.
The
CPU
outputs control signals
to
the squelch
control gate
when
the
S-meter
signal
is
at
a
low
level.
WFM
mode
The
WFM
demodulator
(IC2) controls input
voltage
of the
meter
amplifier (IC9a, pin 2) via
Q6
and
the
AGC
line.
The
same
circuit is
used even
in
WFM
mode
with
SSB,
CW
and
AM-N
modes.
•
Squelch and
AF
amplifier circuits
4-4