Circuit Description - Kenwood TK-260N Service Manual

TK-260G/(N)/270G/(N)
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 : 136~150MHz ((N)E2)
TX/RX : 146~174MHz (E, (N)E)
MCF
ANT RF
SW AMP
49.95MHz
RX : 185.95~199.95MHz ((N)E2)
RX : 195.95~223.95MHz (E, (N)E)
PA TX
AMP AMP
TX : 136~150MHz ((N)E2)
TX : 146~174MHz (E, (N)E)
Fig. 1 Frequency configuration
2. Receiver
The frequency configuration of the receiver is shown in Fig. 2.
L307 RF
D303 AMP
ANT
BPF Q301
ANT
SW
TUNE
CF1 : Wide E
Narrow (N)E,(N)E2,(N)T
IC4
IF, MIX, DET
2nd Local
X3 multiply
Q12
2 1
IC14
HPF LPF HPF
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.
18

CIRCUIT DESCRIPTION

CF
450kHz
IF AF
SP
SYSTEM AMP
50.4MHz
X3
multiply
TCXO 16.8MHz
1/2
PLL
MIC
MIC
DIVIDER VCO
AMP
RX : 371.9~399.9MHz ((N)E2)
RX : 391.9~447.9MHz (E, (N)E, (N)T)
TX : 272~300MHz ((N)E2)
TX : 292~348MHz (E, (N)E, (N)T)
L306,305
MIXER IF AMP
D302.305 MCF
XF1
BPF Q19 Q22
1st Local
TUNE
1/2 PLL
DIVDER VCO
IC301
AF AMP
AF AMP AF AMP
IC15(2/2) AF VOL IC11
IC16 (2/2)
TCXO
BEF
DE-
EXP MUTE
EMP
41
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) 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 5K/Wide 4K changeover circuit (without : (N)E,
(N)E2, (N)T)
The Wide 5K (high level) and Wide 4K (low level) data is
output from IC5 (OUTPUT EXPANDER), pin 4.
Q23 turns on/off with the Wide 5K/Wide 4K data and the
IC4 detector output level is changed to maintain a constant
output level during wide or narrow signals.
5R
SP
AFO
Fig. 3 Wide 5K/Wide 4K changeover circuit
Q23
R105
Wide 5K/Wide 4K
Q22
IC4
FM IF SYSTEM
"H" : Wide 5K
"L" : Wide 4K
CF1