Circuit Description - Kenwood TK-2118 Service Manual

1. Frequency configuration
The receiver utilizes double conversion. The first IF is
51.65MHz and the second IF is 450kHz. 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.
2. Receiver
The receiver is double conversion superheterodyne,
designed to operate in the frequency range of 150 to 174MHz
(C type), 136 to 150MHz (C2 type).
The frequency configuration is shown in Fig. 1.
1) Front - end RF amplifier
An incoming signal from the antenna is applied to an RF
amplifier (Q20) after passing through a transmit/receive switch
circuit (D24, D25, D26, AND D27 are off) and a band pass
filter (L44, L47 and L49). After the signal is amplified (Q20),
the signal is filtered through a band pass filter (L34 and L40) to
eliminate unwanted signals before it is passed to the first
mixer. Band pass filters (L44, L47, L49, L34 and L40) have
varactor diodes (D21, D22, D23, D16 and D20).
The voltage of these diodes are controlled by to track the
MPU (IC300) center frequency of the band pass filter. (See
Fig. 2)
ANT
D24, D25
BPF
D26, D27
ANT SW
TUNE
APC
IC300
MPU
TK-260 :K, K2

CIRCUIT DESCRIPTION /

ANT
RF
ANT SW
AMP
51.65MHz
RX
PA TX
AMP
AMP
TX
Fig. 1 Frequency configuration / 图1 电路构成
RF AMP MIXER
Q20 BPF Q12
1st Local OSC
(PLL)
Fig. 2 Receiver section configuration / 图2 接收部构成
电路说明
1. 频率构成
接收部采用二次变频超外差方式。第一中频为51.65MHz·第
二中频为4 5 0 K H z 。第一本振频率信号由锁相环电路( P L L ) 提
供。
发射部由锁相环电路直接产生所需要的频率。图１显示各种频
率。
CF
450kHz
MCF
AF
IF SYSTEM
AMP
51.2MHz
X4
multiply
TCXO 12.8MHz
PLL MIC
VCO AMP
2.接收部
接收部为二次变频超外差方式，设计操作的频率范围是1 5 0 -
174MHz (C)，136-150MHz (C2)。
1) 前端射频放大器
从天线输入的信号经过收发转换电路 (D24, D25, D26和D27断
开) 和带通滤波器 (L44, L47和L49) 后，在射频放大器 (Q20) 处放
大。信号被放大后 ( Q 2 0 )，在通过第一混频器之前，经过带通
滤波器 (L34和L40) 滤波来消除不要的信号。带通滤波器(L44,
L47, L49, L34和L40) 有变容二极管 (D21, D22, D23, D16和D20)。
这些变容二极管的电压由带通滤波器的MPU (IC300) 中心频
率控制。(参见图2)
MCF IF AMP
XF1 Q6
IF, MIX, DET
TCXO
TK-2118
SP
MIC
CF1
AF PA AMP
IC309
AF AMP
LPF, HPF
IC2
IC306
X4
multiply
Q2
WIDE/NARROW SW
Q316
SP
11