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
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