Kenwood NEXEDGE NX-820 Service Manual Download

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NX-820

1. Overview

The NX-820 is a UHF Mobile transceiver designed to

operate in the frequency range of 400 to 470MHz. 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 4

5MHz 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 4

5MHz. 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.

CIRCUIT DESCRIPTION /

电路说明

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 and 210). 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
amplifier (Q202), and passed through the bandpass filter
(L207, 208, 20

and 211). 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 (4

5MHz).

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).

Fig.1 Frequency confi guration /

图 1 频率构成

ANT

AMP

MIX

IF AMP

Baseband Circuit

AMP

MIC

AMP

PLL

VCO

TCXO

AMP

TX / RX : 400~470MHz

ANT

1st MIX

MCF

450kHz

49.95MHz

16. 8 MHz

× 3

50.4 MHz

MIC

400~470MHz

350.05~

420.05MHz

1. 概述

NX-820 为 UHF 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 (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 49.95M H z 的中频。然
后与第二个 50.4M 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。

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 和 210)。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  ( I C712) 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、208、209 和
211)。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 )。