Icom IC-24AT Service Manual page 6

SECTION
3 CIRCUIT
DESCRIPTION
3-1
VHF RECEIVER
CIRCUITS
3-1-1
ANTENNA
SWITCHING
CIRCUIT
(HPF
UNIT)
The
received signals that
pass
through
the
LPF UNIT
are
separated
as
VHF
and
UHF
signals
on
the
HPF
UNIT
and
are
applied
to
each
RF
amplifier
circuit.
The
HPF
UNIT
contains a
high-pass
filter,
low-pass
filter
and
an antenna
switching
circuit.
The
VHF
signals
pass
through
the
low-pass
filter
(L3-L5,
C6~C10)
and
the
antenna
switching
circuit
(C12~C14,
L6, L7, D2).
The
VHF
signals
and
are then applied
to the
V*RF
UNIT.
The
high-pass
filter
(C1~C5,
LI, L2)
is
designed
for
UHF
receiving
and does
not
allow
VHF
signal
passage.
3-1-2
RF
AND
1ST MIXER CIRCUITS
(V.RF UNIT)
The
RF
and
1st
mixer
circuits
are located
on
the
V»RF UNIT
where
the signals are amplified
at
Q1
and
Q2
and
are
then
converted
to
a
1st IF
signal
at
Q3.
The
frequency
of
the
1st
IF signal
is
30.875
MHz.
Q3
converts the
RF
signal using the
V*PLL
output
signal
coming from
the
"LLO"
line.
The
VHF RF
circuit
adopts a tuned
bandpass
filter
circuit that
tunes the
filters to
the center frequency
of
the
receiving
signal
using varactor diodes
(D2, D4, D5).
A
PLL
lock
voltage
is
used
for
the
filter
tuned
signal.
3-1-3
IF
CIRCUIT (DET
UNIT)
The
1st IF signal
passed
through the
crystal
filter
(F11
on
the
MAIN
UNIT)
enters the
DET
UNIT.
The
DET
UNIT
converts
the
1st IF
signal
to
a 2nd
IF
signal
and
then detects
AF
signals.
The
unit
also
has a
squelch
circuit
that
is
explained
in
Section 3-1
-5.
The
1
St
IF signal
amplified
at
Q3
is
applied
to
the
FM
IF
1C
(IC1
,
pin20). IC1
contains
mixer,
oscillator, limiter amplifier,
quadrature detector
and
meter
detector
circuits.
IC1
converts the entered
signal
from
pin
20
to
a
455 kHz
2nd
IF signal
using the
oscillated
signal
and
outputs the
2nd
IF
from
pin
4
to
pass
through
the crystal
filter (FI1
).
The 2nd
IF signal re-enters IC1 (pin8)
where
it
is
converted
to
an
AF
signal
at
a quadrature
detector (IC1
,
XI
).
The
AF
signals are
output from
IC1 (pinll)
and
are
de-emphasized
with
-6
dB/oct.
at
the integrating
circuit
(R8,
C28, C29) and
are
then
applied
to the
AF
UNIT.
3-1-4
AF
CIRCUIT
(AF UNIT)
The
AF
UNIT
contains the
AF
drive amplifier
(Q1
,
Q2),
power
amplifier (IC1),
noise squelch switch (Q4~Q6) and
tone
squelch
switch
(Q3).
The
AF
signals
from the
DET
UNIT
are
amplified
at
the
AF
drive amplifier
and
are
passed
through the [VOL]
control
on
the
VR
UNIT
and
are then amplified
at
the
power
amplifier
(IC1) to obtain the
speaker
driving level.
3-1-5
SQUELCH
CIRCUIT
(DET
AND
AF
UNITS)
In
an
FM
receiver,
noise signals are output from the speaker
when
no
signal
is
received and
are
suppressed
when
a
signal
is
received.
The
noise
squelch
circuit
cuts
off
the
noise signals using
this
phenomenon.
Noise components
in
AF
signals
(20
kHz and
above) are
amplified
at
the active
filter
(IC1,
RIO, R11, Cl
4,
C15
on
the
DET
UNIT) and
are
rectified at
D1
to convert
to
DC
voltages.
These
voltages switch Q2. Thus, while receiving no
signal,
the
"LOW"
signal
is
output from the
DET
UNIT
as
the
"BUSY"
signal.
If
the
"BUSY"
signal
goes down,
the
"AFS"
signal
goes
down.
The
"AFS"
signal controls the
power
source
(Q4-Q6)
of
the
AF
power
amplifier (IC1). Thus,
when
the
"BUSY"
signal
is
at
"LOW,"
the
AF
power
amplifier
does
not
operate.
When
an
optional
tone squelch
unit
is
installed,
05
on
the
MAIN UNIT
controls the "AFS"
signal using the
"TSQL"
signal.
Fll
IF
CIRCUIT
3-1