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
MCF
ANT RF
SW AMP
49.95MHz
RX : 199.95~223.95MHz
1/2
PA TX
DIVIDER
AMP
AMP
TX : 150~174MHz
Fig. 1 Frequency configuration
2. Receiver
The frequency configuration of the receiver is shown in Fig. 2.
L307 RF L306,305
D303 AMP D302.305
ANT
BPF Q301 BPF
ANT
SW
TUNE TUNE
CF1 : Wide
CF2 : Narrow
AF AMP
IC16 (2/2)
IC4
IF, MIX, DET
2nd Local
X3 multiply
TCXO
Q12
2 1
IC14
HPF LPF HPF BEF
5
Fig. 2 Receiver section
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.

CIRCUIT DESCRIPTION

CF
450kHz
IF AF
SP
SYSTEM AMP
50.4MHz
X3
multiply
TCXO 16.8MHz
PLL
MIC
MIC
VCO
AMP
RX : 399.9~447.9MHz
TX : 300~348MHz
MIXER IF AMP
MCF
XF1
Q19 Q22
1st Local
1/2 PLL
DIVDER VCO
IC301
AF AMP AF AMP
IC15(2/2) AF VOL IC11
SP
DE-
EXP MUTE
EMP
41
TK-260G/270G
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
450kHz.
The second IF signal is passed through the ceramic filter
(CF1; Wide, CF2 ; Narrow) to remove the adjacent channel
signal. The filtered second IF signal is amplified by the
limiting amplifier and demodulated by the quadrature
detector with the ceramic discriminator (CD1). The
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
received, Q14 turn on and Q17 turn off. D14 and D13 are
switched to ceramic filters when a high/low level data is
received.
Q23 turns on/off with the Wide/Narrow data and the IC4
detector output level is changed to maintain a constant
output level during wide or narrow signals.
5R
R105
Q22
AFO
5R
Q14
D14
Q17
Fig. 3 Wide/Narrow changeover circuit
Q23
W/N
IC4
FM IF SYSTEM
"H" : Wide
W/N
"L" : Narrow
CF1
D13
CF2
17