Circuit Description - Kenwood TK-260G Service Manual

1. Frequency configuration
The receiver utilizes double conversion. The first IF is 49.95
MHz and the second IF is 450 kHz. The first local oscillator
signal is supplied form the PLL circuit.
The PLL circuit in the transmitter generates the necessary
frequencies. Fig. 1 shows the frequencies.
ANT
TX/RX : 150~174MHz
ANT RF
SW AMP
PA TX
AMP AMP
TX : 150~174MHz
Fig. 1 Frequency configuration
2. Receiver
The frequency configuration of the receiver is shown in Fig. 2.
ANT
ANT
SW
TUNE
IC4
IF, MIX, DET
X3 multiply
Q12
2
IC14
HPF LPF
5
1) Front end (RF AMP)
The signal coming form the antenna passes through the
transmit/receive switching diode circuit, (D3,D7) passes
through a BPF(L307), and is amplified by the RF amplifier
(Q301).
The resulting signal passes through a BPF (L305 and L306)
and goes to the mixer. These BPFs are adjusted by variable
capacitors (D302, 303,305). The input voltage to the
variable capacitor is regulated by voltage output from the
microprocessor (IC13)
2) First mixer
The signal from the front end is mixed with the first local
oscillator signal generated in the PLL circuit by Q19 to
produce a first IF frequency of 49.95 MHz.
The resulting signal passes through the XF1 MCF to cut
the adjacent spurious and provide the opitimun
characteristics, such as adjacent frequency selectivity.
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CIRCUIT DESCRIPTION

CF
450kHz
MCF
IF
SYSTEM
49.95MHz
50.4MHz
X3
multiply
RX : 199.95~223.95MHz
1/2
PLL
DIVIDER VCO
RX : 399.9~447.9MHz
TX : 300~348MHz
L307 RF L306,305
MIXER IF AMP
D303 AMP D302.305 MCF
BPF Q301 BPF Q19 XF1 Q22
1st Local
TUNE
1/2
DIVDER
IC301
CF1 : Wide
CF2 : Narrow
AF AMP
AF AMP
IC15(2/2)
AF VOL
IC16 (2/2)
2nd Local
TCXO
1
HPF BEF
DE-
MUTE
EXP
EMP
Fig. 2 Receiver section
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3) IF Amplifier circuit
The first IF signal is passed through a four-pole monolithic
crystal filter (XF1) to remove the adjacent channel signal.
The filtered first IF signal is amplified by the first IF amplifier
(Q22) and then applied to the lF system IC (IC4). The IF
system IC provides a second mixer, second local oscillator,
limiting amplifier, quadrature detector and RSSI (Received
Signal Strength Indicator). The second mixer mixes the
first IF signal with the 50.4MHz of the second local oscillator
output (TCXO X3) and produces the second IF signal of
AF
SP
450kHz.
AMP
The second IF signal is passed through the ceramic filter
(CF1; Wide, CF2 ; Narrow) to remove the adjacent channel
TCXO 16.8MHz
signal. The filtered second IF signal is amplified by the
limiting amplifier and demodulated by the quadrature
detector with the ceramic discriminator (CD1). The
MIC
MIC
AMP
demodulated signal is routed to the audio circuit.
4) Wide/Narrow changeover circuit
Narrow and Wide settings can be made for each channel
by switching the ceramic filters CF1 (Wide) and CF2
(Narrow).
The WIDE (high level) and NARROW (low level) data is
output from IC5 (OUTPUT EXPANDER), pin 4.
When a WIDE (high level) data is received, Q14 turn off
and Q17 turn on. When a NARROW (low level) data is
PLL
received, Q14 turn on and Q17 turn off. D14 and D13 are
VCO
switched to ceramic filters when a high/low level data is
received.
Q23 turns on/off with the Wide/Narrow data and the IC4
AF AMP
detector output level is changed to maintain a constant
IC11
SP
output level during wide or narrow signals.
41
5R
Fig. 3 Wide/Narrow changeover circuit
TK-260G/270G
5R
R105
Q22
IC4
AFO
FM IF SYSTEM
W/N
Q14 CF1
D14
CF2
Q17
Q23
W/N
"H" : Wide
"L" : Narrow
D13
17