Circuit Description; 电路说明 - Kenwood TK-3148 Service Manual

TK-3148
1. Overview
This transceiver is UHF/FM portable transceiver designed
to operate in the frequency range of 350 to 390MHz (C).
2. Circuit Confi guration by Frequency
The receiver is a double-conversion superheterodyne
with a fi rst intermediate frequency (IF) of 44.85MHz and a
second IF of 455kHz. Incoming signals from the antenna are
mixed with the local signal from the PLL to produce the fi rst
IF of 44.85MHz.
This is then mixed with the 44.395MHz second local
oscillator output to produce the 455kHz second IF. This is
detected to give the demodulated signal.
The transmit signal frequency is generated by the PLL
VCO, and modulated by the signal from the microphone. It
is then amplifi ed and sent to the antenna.
3. Receiver System
3-1. RF unit
An incoming RF signal from the antenna terminal is
passed through the antenna switch (D102, D103, D104 and
D105 are off) and then the bandpass fi lter (L215 and L217).
High pass fi lter HPF (L219) the 1st image response improve.
And the bandpass fi lter is adjusted by a variable capacitor.
The input voltage to the variable capacitor is regulated by
the voltage output from the D/A converter (IC307). The sig-
nal is amplifi ed by RF amplifi er Q207, and passed through
the bandpass filter (L209, L210 and L211). The resulting
signal is applied to the fi rst mixer (Q206), where it is mixed
with the fi rst local oscillator signal output from the frequen-
cy synthesizer to produce the fi rst IF (44.85MHz).
3-2. IF unit
The fi rst IF signal is passed through a crystal fi lter (XF200)
to remove a adjacent channel signal. The fi ltered fi rst IF sig-
nal is amplifi ed by the fi rst IF amplifi er (Q205) and then ap-
plied to the lF system IC (IC200). 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 fi rst IF signal with
the 44.395MHz of second local oscillator output (crystal unit
X200) and produces the second IF signal of 455kHz.
10

CIRCUIT DESCRIPTION /

TX/RX: 350~390MHz (C)
ANT
1st MIX
RF
ANT
AMP
SW
305.15~345.15MHz (C)
PA TX
AMP
AMP
350~390MHz (C)
Fig. 1 Frequency confi guration / 图 1 频率构成
电路说明
1. 综述
本对讲机是在 350 到 390MHz(C) 的频率范围内工作的 UHF/
FM 对讲机设备。
2. 电路的频率构成
接收部采用 44.85M H z 的第一中频和 455k H z 的第二中频的
二次变频超外差方式。通过天线接收到的信号与来自于锁相
环电路的本振信号混频生成 44.85MHz 的第一中频。
然后与 44.395MHz 的第二本振信号混频生成 455kHz 的第二
中频。此频率被检测并产生解调信号。
锁相环压控振荡器生成发射信号频率，并且被来自于麦克
风的信号调制。然后被放大并发送到天线。
CF
455kHz
MCF
FM IF
SYSTEM
44.85MHz
44.395MHz
PLL
VCO
3. 接收部系统
3-1. 射频单元
由天线接收到的射频信号进入天线开关 (D102，D103，D104
和 D105 截止 )，然后通过带通滤波器 ( L215 和 L217)。高通
滤波器 H P F ( L219)，抑制镜像信号。带通滤波器由可变电容
进行调整。
向可变电容输出的电压通过来自于数 / 模转换器 ( I C307)
的电压输出进行调节。信号被射频放大器 Q207 放大，并通过
带通滤波器 ( L209，L210 和 L211) 生成的信号进入第一混频
器 ( Q206)，并且与来自于频率合成器的第一本振信号混频生
成第一中频 (44.85MHz)。
3-2. 中频单元
第一中频信号通过晶体滤波器 ( X F200) 消除邻道的信号。
经滤波的第一中频信号被第一中频放大器 ( Q205) 放大并进入
中频系统芯片 (IC200)。中频系统芯片提供第二混频器，第二
本振信号，限幅放大器，正交检测器和 R S S I ( 接收信号强度
指示器 )。第二混频器将第一中频信号与 44.395MHz 的第二本
振信号输出 ( 晶体单元 X200) 进行混频，并生成 455kHz 的第
二中频信号。
AF
SP
AMP
MIC
MIC
AMP