Section 4 Circuit Description; Receiver Circuits - Icom IC-T7A Service Manual

SECTION
4
CIRCUIT
DESCRIPTION
4-1
RECEIVER
CIRCUITS
4-1
-1
DUPLEXER
CIRCUIT
(1
F
UNIT)
The
transceiver
has a duplexer (low-pass
and
high-pass
filters)
on the
first
stage from the antenna connector
to
separate the signals
into
VHF
and
UHF
signals.
The
low-pass
filter
(L10-L12,
C16-C22)
for
VHF
signals
and
high-pass
fiiter
(L1-L3,
C1-C5)
for
UHF
signals.
The
separated signals
are applied
to
each
RF
circuit.
4-1-2
VHF
ANTENNA
SWITCHING
CIRCUIT
{IF
UNIT)
The
antenna switching
circuit
functions
as a low-pass
filter
while receiving.
However,
its
impedance becomes
very
high while transmitting
by applying a
current to
D51 and
D52. Thus, transmit signals
are blocked
from
entering the
receiver
circuits.
The
antenna
switching circuit
employs a
1/4
A
type diode switching system.
The
passed
signals
are
then
applied to
the
RF
amplifier circuit
on
the
2F
unit.
4-1-3
VHF
RF CIRCUIT
(2F
UNIT)
The
RF
circuit
amplifies signals within the
range
of
fre-
quency coverage and
filters
out-of-band
signals.
The
signals from the
antenna
switching circuit
are applied
to
the
bandpass
filter
(D52,
L53),
and are then applied
to
the
RF
amplifiers (Q51, Q52).
The
amplifier consists of
a
cascade
circuit.
The
amplified signals
are
passed
through
the next stage
bandpass
filter
(D53,
D54, L54, L55)
to
suppress
unwanted
signals.
The
filtered
signals are then
applied
to
the
mixer
circuit
(Q401).
D53 and
D54
track the
bandpass
filters
and
are
controlled
by
the
PLL
lock voltage.
These
diodes tune the center
frequency
to
obtain
good image response
rejection.
4-1-4
UHF RF
CIRCUIT
(2F UNIT)
The
signals
from the
antenna
switching circuit
(IF
unit
D551, D552)
are ampiified at
the the
RF
amplifier (Q201)
and are then passed though the
bandpass
filter
(FI201).
The
filtered
signals
are applied
to
another
RF
amplifier
(Q202)
and
are then applied
to
the 1st
mixer
circuit
(Q401).
Common
circuits
with
VHF
band
are
used
later
stage from
the
1
St
mixer.
4-1 -5
1
ST MIXER
AND
1
ST
IF
CIRCUITS
(2F
UNIT)
The
mixer
circuit
converts the received signal to
a
fixed
frequency
of the
1
st
IF signal with
a
1
st
LO (VCO
output)
frequency.
By
changing
th
PLL
frequency, only the desired
frequency
will
be passed though a
crystal
filter
at
the next
stage
of
the
mixer.
The
receive signals
from the
VHF
or
UHF RF
circuit
are
mixed
with
the
1
st
LO
signal
(VCO
output signal) at the
1
st
mixer (0401)
to
produce a 45.15
MHz
1st IF signal.
The
1st IF signal
is
applied to
a
crystal
filter
(FI401) to
suppress out-of-band
signals.
The
filtered
1
st
IF signal
is
amplified
at
the IF amplifier (01 01)
and
is
then applied to
the
2nd mixer
circuit
(IC101).
4-1-6
2ND
IF
AND DEMODULATOR
CIRCUITS
(2F
UNIT)
The
2nd mixer
circuit
converts the 1st IF signal
to
a 2nd
IF
signal.
A
double superheterodyne system (which converts
receive signals twice)
improves the
image
rejection ratio
and
obtains stable receiver gain.
The
FM
IF
1C (IC101) contains the
2nd
mixer,
2nd
local
oscillator, limiter amplifier,
S-meter
detector
and
quadrature
detector
circuits.
The
1st IF signal (45.15
MHz)
from
the IF amplifier (01 01)
is
applied to
the
2nd
mixer section of IC101
(pin 16),
and
is
mixed
with the
2nd LO
signal (45.6
MHz)
for
conversion
to
a
450 kHz 2nd
IF signal at the
2nd mixer
section.
The
2nd
IF signal
(450 kHz) from the
2nd mixer
section
(IC101
pin
3)
passes though
the
ceramic
filter
(F1
101)
where unwanted
signals are
suppressed.
It
is
then
amplified
at
the
limiter
amplifier
section (IC101
pin 5)
and
applied
to
the
quadrature detector section
to
demodulate
the
2nd
IF signal
into
AF
signals.
AF
signals output
from IC101
(pin
9)
are applied
to
the
AF
drive amplifier
(012)
on the
LOGIC
unit.
The
S-meter
output
"SD"
signal
from IC101
(pin
12) is
applied
to
the
CPU
(LOGIC
unit
pin
3).
2nd
IF
AND DEMODULATOR
CIRCUIT
2nd
IF
filter
450 kHz
4-1