Circuit Description - Kenwood NX-720 Service Manual

1. Overview
The NX-720 is a VHF Mobile transceiver designed to
operate in the frequency range of 136 to 174MHz. The unit
consists of a receiver, transmitter, phase-locked loop (PLL)
frequency synthesizer, baseband parts, power supply, and
control circuits.
2. Frequency Confi guration
The receiver is a double-conversion super-heterodyne
using the fi rst intermediate frequency (IF) of 49.95MHz and
the second IF of 450kHz. Incoming signals from the an-
tenna are mixed with the local signal from the PLL circuit to
produce the fi rst IF of 49.95MHz. This is then mixed with
the 50.4MHz second local oscillator output to produce the
450kHz second IF. The transmit signal frequency is gener-
ated by the PLL VCO, and modulated by the signal from the
DSP. It is then amplifi ed and fed to the antenna.
3. Receiver System
3-1. RF circuit
An incoming RF signal from the antenna terminal is
passed through the antenna switch (D110, 111 and 107)
and then the bandpass fi lter (L215, 216). 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 (IC712). The signal is amplifi ed by an RF am-
plifi er (Q202), and passed through the bandpass fi lter (L207,
209, 210 and 220). The resulting signal is applied to the fi rst
mixer (Q201) where it is mixed with the fi rst local oscillator
signal output from the frequency synthesizer to produce the
fi rst IF (49.95MHz).
3-2. IF circuit
The fi rst IF signal is passed through a four-pole mono-
lithic crystal fi lter (XF1) to reject the adjacent channel signal.
The fi ltered fi rst IF signal is amplifi ed by the fi rst IF amplifi er
(Q305) and then applied to the IF system IC (IC303). The
IF system IC provides a second mixer, AGC amplifi er , and
RSSI (Received Signal Strength Indicator).
CIRCUIT DESCRIPTION / 电路说明
ANT
TX/RX:136~174MHz
1st
MIX 49.95MHz
ANT RF
SW AMP
16.8MHz
TCXO
185.95~
223.95MHz
POWER TX
AMP AMP
136~174MHz
Frequency confi guration / 图 1 频率构成
Fig.1
1. 概述
N X -720 为 V H F 车载对讲机，设计用于 136 至 174M H z 的频
率范围。 该设备由接收机、 发射机、 锁相环 (PLL) 频率合成器、
基带部件、电源和控制电路组成。
2. 频率构成
接收机为二次变频超外差方式，使用的第一中频 ( IF ) 为
49.95MHz，第二中频为 450kHz。天线接收到的信号与 PLL 电
路的本振信号混频，从而产生第一个 49.95MHz 的中频。然后
与第二个 50.4M Hz 的本地振荡器输出信号混频，进而产生第
二个 450kHz 的中频。发射的信号频率由 PLL VCO 生成，并经
DSP 信号调制。最后，该信号频率经过放大并发送到天线。
CF 450kHz
MCF
IF SYSTEM
X3
50.4MHz
PLL/VCO
3. 接收部系统
3-1. RF 电路
天线端子接收到的 RF 输入信号依次通过天线开关 (D110、
111 和 107) 和带通滤波器 (L215、216)。带通滤波器经由
可变电容器调整。可变电容器的输入电压经由数模转换器
(IC712) 的输出电压进行调整。 随后， 信号被 RF 放大器 (Q202)
放大，然后通过带通滤波器 (L207、209、210 和 220)。最终
的信号被运用到第一混频器 (Q201)。在该混频器中，信号与
频率合成器输出的第一本地振荡器信号混频，从而产生第一
中频 (49.95MHz)。
3-2. 中频电路
第一中频信号经过一个四极单片晶体滤波器 (XF1) 以消除
临近信道的信号。滤波后的第一中频信号经过第一中频放大
器 (Q305) 的放大后，加载到中频系统 IC (IC303) 上。中频
系统 IC 提供一个第二混频器、AGC 放大器和 RSSI ( 接收信号
强度指示器 )。
NX-720
SP
AF
PA
MIC
MIC
AMP
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