Circuit Description - Kenwood TK-2168 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 from the PLL circuit.
The PLL circuit in the transmitter generates the necessary
frequencies. Fig. 1 shows the frequencies.
ANT TX/RX: 136 ~ 174MHz
RF
ANT SW
AMP
49.95MHz
RX: 185.95 ~ 233.95MHz
TX: 136 ~ 174MHz
TX
AMP AMP
Fig. 1 Frequency configuration
2. Receiver
The frequency configuration of the receiver is shown in Fig. 2.
ANT
RF AMP
Q404
BPF
ANT SW
TUNE
CF401
IC602
AF Amp
IC401
IF,MIX,DET
X1
Q4
TCXO 16.8MHz
X3 multiply
2nd Local
Fig. 2 Receiver section
1) Front End (RF AMP)
The signal coming from the antenna passes through the
transmit/receive switching diode circuit, (D204,D206,D208
and D212) passes through a BPF (L411 and L412), and is
amplified by the RF amplifier (Q404).
The resulting signal passes through a BPF (L407 and L409)
and goes to the mixer. These BPFs are adjusted by variable
capacitors (D402,D403,D404 and D405). The input voltage
to the variable capacitor is regulated by voltage output from
the microprocessor (IC805).
2) First Mixer
The signal from the front end is mixed with the first local
oscillator signal generated in the PLL circuit by Q403 to
produce a first IF frequency of 49.95 MHz.
The resulting signal passes through the XF401 MCF to cut
the adjacent spurious and provide the opitimun
characteristics, such as adjacent frequency selectivity.

CIRCUIT DESCRIPTION

CF
450kHz
MCF
AF
IF SYSTEM
AMP
50.4MHz
X3 multiply
TCXO
16.8MHz
RF PLL
MIC
VCO
AMP
MIXER MCF
IF AMP
Q403 XF401
BPF Q402
1st Local
TUNE
IC601
IC605
AF VOL AF PA
AQUA
3) IF Amplifier Circuit
The first IF signal is passed through a four-pole monolithic
crystal filter (XF401) to remove the adjacent channel signal.
The filtered first IF signal is amplified by the first IF amplifier
(Q402) and then applied to the lF system IC (IC401). 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
SP
local oscillator output (TCXO X1) and produces the second
IF signal of 450kHz.
The second IF signal is passed through the ceramic filter
(CF401) to remove the adjacent channel signal. The filtered
second IF signal is amplified by the limiting amplifier and
MIC
demodulated by the quadrature detector with the ceramic
discriminator (CD401). The demodulated signal is routed
to the audio circuit.
4) Wide/Narrow Switching Circuit
Narrow and Wide settings can be made for each channel
by switching the demodulation level.
The WIDE (high level) and NARROW (low level) data is
output from IC805, pin 54.
When a WIDE (high level) data is received, Q401 turn off.
When a NARROW (low level) data is received, Q401 turn on.
Q401 turns on/off with the Wide/Narrow data and the
IC401 detector output level is switched to maintain a
constant output level during wide or narrow signals.
SP
AFOUT
Q401
W/N
H : Wide
L : Narrow
Fig. 3 Wide/Narrow switching circuit
5) Audio Amplifier Circuit
The demodulated signal from IC401 is amplified by IC602,
and goes to AF amplifier through IC601.
The signal then goes through an AF volume control
(VR801), and is routed to an audio power amplifier (IC605)
where it is amplified and output to the speaker.
TK-2168
Q402
IC401
FM IF SYSTEM
IFO
QUAD
C409
R408
CD401
R409
5R
7