Circuit Description; 电路说明 - Kenwood NX-800 Service Manual

1. Overview
NX-800 is a UHF Mobile transceiver designed to operate
in the frequency range of 400 to 470MHz. The unit consists
of receiver, transmitter, phase-locked loop (PLL) frequency
synthesizer, base band parts, power supply, and control cir-
cuits.
2. Frequency Confi guration
The receiver is a double-conversion super heterodyne
using first intermediate frequency (IF) of 58.05MHz and
second IF of 450kHz. Incoming signals from the antenna
are mixed with the local signal from the PLL circuit to pro-
duce the first IF of 58.05MHz. This is then mixed with
the 57.6MHz 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.
ANT
ANT
SW
PA
AMP
3. Receiver System
3-1. RF circuit
The front-end circuit consists of former BPF (D212 and
D213), RF amplifier Q210, and latter BPF (D207, D208,
D209 and D210).
The BPF covers frequency ranges 400 to 470MHz.
The latter BPF (D207, D208, D209 and D210) attenuates
the unwanted signals, and sends only the necessary signal
to the fi rst mixer.
3-2. First Mixer
The signal from the BPF is heterodyned with the fi rst lo-
cal oscillator signal from the PLL frequency synthesizer cir-
cuit at the fi rst mixer DBM (IC204) to become a 58.05MHz
fi rst intermediate frequency (IF) signal.
The fi rst IF signal is fed through a monolithic crystal fi lter
(XF202) to further remove spurious signals.

CIRCUIT DESCRIPTION /

TX/RX: 400~470MHz
1st MIX MCF
RF
AMP
58.05MHz
19.2MHz
VCTCXO
341.95~
411.95MHz
TX
AMP
400~470MHz
Fig. 1 Frequency confi guration / 图 1 频率构成
电路说明
1. 概述
NX-800 为 UHF 便携式车载对讲机，设计用于 400 至 470MHz
的频率范围。该设备由接收机、发射机、锁相环 ( P L L ) 频率
合成器、基带部分、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 ( I F ) 为
58.05MHz，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 58.05M H z 的中频。然后
与第二个 57.6M H z 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
MIX AF
IF AMP AMP
x3
57.6MHz
PLL MIC
VCO AMP
3. 接收部系统
3-1. RF 电路
前端电路由前级 B P F ( D212 和 D213)、R F 放大器 Q210 和末
级 BPF(D207、D208、D209 和 D210) 组成。
BPF 覆盖的频率范围为 400 ～ 470MHz。
末级 B P F ( D207、D208、D209 和 D210) 衰减不需要的信号，
然后仅将必要的信号发送给第一混频器。
3-2. 第一个混频器
B P F 的信号与 P L L 频率合成器电路的第一本地振荡器信号
在第一混频器 D B M ( I C204) 进行外差，成为 58.05M H z 的第一
中频 (IF) 信号。
第一 IF 信号经由单片晶体滤波器 (XF202) 馈通以进一步消
除杂散信号。
NX-800
SP
MIC
27