Circuit Description - Kenwood FM Transceiver TK-481 Service Manual

TK-480/481
1. Overview
This transceiver is an 800/900MHz band EFJ LTR™
trunked system compatible FM portable transceiver that can
be programmed to operate on both LTR and conventional
systems.
2. Circuit Configuration by Frequency
The receiver is a double-conversion superheterodyne
with a first intermediate frequency (IF) of 44.85MHz and a
second IF of 455kHz. Incoming signals from the antenna are
mixed with the local signal from the PLL to produce the first
IF of 44.85MHz.
This is then mixed with the 44.395MHz second local os-
cillator output to produce the 455kHz second IF. This is de-
tected to give the demodulated signal.
The transmit signal frequency is generated by the PLL
VCO, and modulated by the signal from the microphone. It
is then amplified and sent to the antenna.
TK-480 TK-481
TX 806~825MHz TX 896~902MHz
851~870MHz 935~941MHz
RX 851~870MHz RX 935~941MHz
ANT
1st MIX
ANT
RF
SW AMP
TK-480 :
806.15~825.15MHz
TK-481 :
890.15~896.15MHz
PA TX
AMP AMP
TK-480
806~825MHz
851~870MHz
Fig. 1 Frequency configuration
ANT
L30 D9,D10
ANT
SW
2
IC12
HPF
5
14

CIRCUIT DESCRIPTION

CF
455kHz
MCF
AF
IF SYSTEM
AMP
44.85MHz
44.395
MHz
PLL
MIC
VCO
AMP
TK-481
896~902MHz
935~941MHz
L11 Q9 L20
BPF RF AMP BPF
1st local
OSC (PLL)
1
LPF HPF BEF
DE-
EMP
Fig. 2 Receiving system
3. Receiver System
3-1. RF unit
An incoming RF signal from the antenna terminal is
passed through the antenna switch (D7, D9, and D10 are
off) and then the bandpass filter (L11). The signal is ampli-
fied by RF amplifier Q9, and passed through the bandpass
filter (L20) to remove the spurious signal again. The result-
ing signal is applied to the first mixer (Q6), where it is mixed
with the first local oscillator signal output from the fre-
quency synthesizer to produce the first IF (44.85MHz).
3-2. IF unit
The first IF signal is passed through a four-pole mono-
lithic crystal filter (XF1) to remove a adjacent channel signal.
The filtered first IF signal is amplified by the first IF amplifier
(Q5) and then applied to the IF system IC (IC9). The IF sys-
tem IC provides a second mixer, second local oscillator, lim-
iting amplifier, quadrature detector and RSSI (Received Sig-
nal Strength Indicator). The second mixer mixes the first IF
signal with the 44.395MHz of second local oscillator output
(crystal unit X1) and produces the second IF signal of
455kHz.
The second IF signal is passed through the ceramic filter
(CF1,2) to more remove the adjacent channel signal. The
filtered second IF signal is amplified by the limiting amplifier
and demodulated by the quadrature detector with ceramic
SP
discriminator (CD1). The demodulated signal is routed to
the audio circuit.
MIC
Q6 XF1 Q5
1st MIX MCF 1st IF
IC9
MIX,DET,IF
X1
2nd local
OSC
IC3 (2/2)
AF AMP
EXP MUTE VOL
41
IC8 (2/2)
AF AMP
CF1
CF2
IC300
AF PA
IC4 Q310
AF AF
SW
VC1
SSW
VC2
INT.
SP
EXT.
SP