Circuit Description; Frequency Configuration - Kenwood TM-321A Service Manual

-321A
CIRCUIT DESCRIPTION
Frequency configuration
The
TM-321A
utilizes
PLL
synthesizer system incorpo-
rating
a
digital
VFO
(See Fig. 1.}.
The channel step can
be
selected
as
5, 10, 15, 20, or
25kHz.
The
receiver operates
as a
double conversion system.
Received signals are mixed
with the first local oscillator
(189.175
~
194.170MHz) to
produce the
first
intermediate
frequency of 30.825MHz. The first intermediate frequency
is
mixed
with the
second local
oscillator (30.370MHz) to
produce the second intermediate frequency of 455kHz.
The transmitter
system consists
of
a
PLL
circuit
in-
corporating a
direct oscillator
and
direct divider. The
output is amplified by a
linear
amplifier prior to
being
transmission.
220~225MHz
220~225MHz
1st
MIX
ANT
SW
RF
AMP
Receiver system
@
General
incoming signals from the antenna
pass through a
low-
pass
filter
in
the
Transmitter Final unit
and
a
diode trans-
mit/receive switch, then enter the receiver front end.
After
passing
through two antenna coils the signals
are
amplified by a
GaAs
(galium arsenide)
FET
(Q1
:
3SK184
(S)).
Undesired signals are removed
by
a 3-pole helical
res-
onator (L3).
The
resulting signal is
applied to the first
mixer Q2
:
3SK131(V12}, which employs
an
N
channel
MOS
FET
to obtain good 2-signal characteristics.
In
the
first mixer (Q2) the
signai is
mixed with the first local
oscillator from the
PLL
system to produce the first IF
signal
of 30.825MHz,
Interfering
Adjacent
channel inter-
ference
is
removed from the
first
IF
signal
by a two-stage
monolithic crystal filter (MCF) (L6).
455kHz
CFW455F
MCF SP
MIX,IF,DET
AF
AMP
30.825MHz
189.175~
+
30.370MHz
194,170MHz
PLL
|
SYSTEM
PA AMP
Tx
amp
1
wic
220,000~
224.995MHz
MIC AMP
Fig. Frequency configuration
2
F
g.