Circuit Description; Receiver Circuits - Icom IC-2GA Service Manual

SECTION
4
CIRCUIT
DESCRIPTION
4
-
1
RECEIVER CIRCUITS
(1)
ANTENNA
SWITCHING
CIRCUIT
(RF
UNIT)
RF signals enter the antenna connector
and pass through
a
series of
Chebyshev
low-pass
filters (LI, L2,
Cl, C2, C6, C7,
C8).
The
signals are
then applied to the
antenna
switching
circuit,
which employs
a
two-stage diode switching
system
(L3, L4,
C4, C5, C11, D1),
(2)
RF CIRCUIT
{RF
UNIT)
The
signals
from the antenna switching
circuit
are ampli-
fied at
Q1. Amplified signals are applied to a
bandpass
filter
(Cl
7,
C22, C25, D3, D5, D6, L6'-L8)
where
out-of-band
signals are
suppressed. D2, D3, D5,
and
D6
are varactor
diodes
which
track
the bandpass
filters
with the
PLL
lock
voltage.
(3)
1st
MIXER
CIRCUIT {RF UNIT)
The
signals are applied to the gate of the
first
mixer
(Q2).
A
frequency
oscillated in
the
PLL passes through
the
LO
switching
circuit
(D16),
and
is
applied to the
source
of Q2.
The
received signals are
mixed
with the
PLL output
to
produce the
first IF
of 16.9
MHz, which
is
picked
up
at L9.
{4)
1st
IF
AMPLIFIER
{RF
UNIT)
The
first IF
signal
is
applied to a set of
two
monolithic
filters
(FI1)
which determine the
bandwidth
(15
kHz/"3 dB) and
suppress
unwanted heterodyned
signals.
The
first IF
signal
is
amplified
at
Q3,
and
then applied to IC1 (pin 16) via C34.
Using XI, IC1 oscillates a
2nd
LO
frequency of 16.445
MHz.
The second
IF
of
455 kHz
is
output from pin
3.
Applied
to
IC1 (pin
5),
the
second
IF
is
amplified
at
the IC's internal
limiter amplifier.
The
amplified signal
is
applied to the
quadrature
detector
(IC1,
pins
7, 8;
X2).
An AF
signal
Is
output
from
the
quadrature detector
through
pin
9.
After
passing through the ceramic
filter
(F12),
a portion of
the
second
IF is
applied to the
S-meter amplifier (Q4, Q5).
{6)
AF
CIRCUIT
{MAIN
UNIT)
The AF
signal
is
applied to
the
de-emphasis
circuit
(C55,
R33),
and then
to the
AF
amplifier (Q219, Q220).
The
de-emphasis
circuit
is
an
integrated circuit
with
frequency
characteristics of
-"6 dB/octave
(300
Hz'-'3 kHz). Passing
through the
volume control (R502), the signals are ampli-
fied
by
1C201,
and sent to the speaker.
{7)
SQUELCH
CIRCUIT
{RF
AND
MAIN
UNITS)
A
portion of the
AF
signal
from
!C1 (pin 9)
is
applied
to pin
10 (the active
filter).
The
SQL
pot (R501) adjusts the input
level at
pin
1.
The
active
filter
in IC1
amplifies noise
components
of
frequencies
of
20 kHz
and
above,
and
outputs
the resulting
signals
from
pin
11.
The
signals
pass
through
D9
for
noise detection.
If
the detected noise
voltage
is
high,
Q18
Is
turned on;
Q202
is
turned
off,
cutting
off
voltage
buffer (Q204,
Q205) output
voltage.
In
this
way,
IC201
is
deactivated
when
the squelch
is
closed.
When
transmitting, a
decrease
In
Q202
voltage cuts off
voltage buffer
output voltage
—
turning
off
IC201,
{5)
2nd
IF
AND DEMODULATOR
CIRCUITS
{RF UNIT)
IC1
contains the
second
local oscillator,
the
second
mixer,
a
limiter amplifier,
and
the
quadrature
detector.
(8)
LOCAL OSCILLATOR CIRCUIT
(RF
UNIT)
Q12
buffer amplifies
the
VCO
output,
which then passes
through D16 and
is
applied
to
the source of Q2.
IF
CIRCUIT
AF
output
to
Q219
4-1