Circuit Description; 电路说明 - Kenwood NX-220 Service Manual

1. Overview
The NX-220 is a VHF portable 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 superheterodyne
using the first intermediate frequency (IF) of 49.95MHz
and the second IF of 450kHz. Incoming signals from the
antenna 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.
ANT
ANT
SW
PA
AMP
3. Receiver System
3-1. RF circuit
An incoming RF signal from the antenna terminal is
passed through the antenna switch (D102, D103, D105 and
D106) and then the band-pass fi lter (L212, L213). The band-
pass fi lter is adjusted by a variable capacitor. The input volt-
age to the variable capacitor is regulated by the voltage out-
put from the D/A converter (IC4). The signal is amplifi ed by
an RF amplifi er (Q202), and passed through the band-pass
fi lter (L207, L208 and L211). The resulting signal is applied
to the fi rst mixer (Q201), where it is mixed with the fi rst lo-
cal 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 monolithic
crystal fi lter (XF1) to reject adjacent channel signals. The fi l-
tered fi rst IF signal is amplifi ed by the fi rst IF amplifi er (Q303)
and then applied to the IF system IC (IC302). The IF system
IC provides a second mixer, AGC amplifi er, and RSSI (Re-
ceived Signal Strength Indicator).

CIRCUIT DESCRIPTION /

TX/RX: 136~174MHz
1st MIX MCF
RF
AMP
49.95MHz
16.8MHz
TCXO
185.95~
223.95MHz
TX
AMP
136~174MHz
Fig. 1 Frequency confi guration / 图 1 频率构成
电路说明
1. 概述
NX-220 为 VHF 便携式手持对讲机，设计用于 136 至 174MHz
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 ( I F ) 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。
CF
450kHz
MIX AF
IF AMP AMP
x3
50.4MHz
PLL MIC
VCO AMP
3. 接收部系统
3-1. RF 电路
天线端子接收到的 R F 输入信号依次通过天线开关 ( D102、
D103、D105 和 D106) 和 带 通 滤 波 器 ( L212、L213)。 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 C4) 的输出电压进行调整。随后，信号被 R F 放大
器 (Q202) 放大，然后通过带通滤波器 (L207、L208 和 L211)。
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 ( X F1) 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 (Q303) 的放大后，加载到中频系统 IC(IC302) 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-220
SP
MIC
17