Kenwood NX-820 Service Manual page 21

4. Transmitter System
4-1. Audio Band Circuit
The signal from the microphone is amplifi ed by IC703 (1/2)
and limited by the AGC circuit composed of D703, D704,
Q703 and Q704. IC703 (2/2) works as an anti-aliasing LPF
fi lter.
4-2. Baseband Circuit
The audio signal output from the Audio band circuit is
converted to digital data with a sampling frequency of 48
kHz. This digital data is sent to the DSP (IC502), and voice
signals of 300Hz or lower and frequencies of 3kHz or higher
are cut off so that an audio range 300Hz to 3kHz is extract-
ed. The audio signal is then pre-emphasized in FM mode
and synthesized with the signals, such as QT and DQT, as
required, and is then output from the ASIC (IC507). In Digi-
tal mode, the audio signal is converted to the 4-Level FSK
baseband signal and is output from IC507. The DTMF and
MSK baseband signals are also generated by the DSP and
output from IC507.
LPF (IC701) works as smoothing fi lter. The output level
according to the transmit carrier is fi ne-adjusted according to
each modulation method.
4-3. Drive and Final amplifi er
The signal from the T/R switch (D17 is on) is amplifi ed
by the drive amplifier (Q102) to 16~17dBm. The output of
the drive amplifi er is amplifi ed by the fi nal amplifi er module
(IC102) to 25W (5.0W when the power is low). The output
of the fi nal amplifi er module is then passed through the har-
monic fi lter (LPF) and antenna switch (D110, D111 are on)
and directional coupler and is applied to the antenna termi-
nal.
4-4. APC circuit
The Automatic transmission power control (APC) circuit
stabilizes the transmitter output power at a predetermined
level by detecting the power module output with the direc-
tional coupler and diode detector (D104 and D105). The di-
ode detector (D104 and D105) applies the detected voltage
to the DC amplifi er IC103 (2/2).
The APC circuit is confi gured to protect over-current of
the power module due to fl uctuations of the load at the an-
tenna end and to stabilize transmission output at voltage and
temperature variations.
CIRCUIT DESCRIPTION / 电路说明
IC 502
MIC IC703 ( 2 / 2 )
AMP
AGC
D703,704
Q703,704
Fig. 4 Transmitter System / 图 4 发射机系统
4. 发射机系统
IC2
PLLIC
DSP
IC701(1 / 2 )
IC507
LPF VCO
ASIC
4-1. 音频频带电路
麦克风信号被 I C703 (1/2) 放大并被 D703、D704、Q703
和 Q704 组成的 AG C 电路所限制。IC703 (2/2) 用作抗混叠
LPF 滤波器。
4-2. 基带电路
从 音 频 频 带 电 路 输 出 的 音 频 信 号 被 转 换 为 取 样 频 率 为
48k H z 的 数 字 数 据。 该 数 字 数 据 被 发 送 给 D S P ( IC502)，
300Hz 或更低的语音信号以及 3kHz 或更高的频率被截止，并
选 取 300Hz 至 3kH z 的 音 频 范 围。 然 后， 音 频 信 号 以 F M 模
式预加重，并与所需的 QT 和 DQT 等信号合成，然后从 AS IC
(I C507) 输出。在数字模式下，音频信号被转换为 4 级 FSK
基带信号并从 IC507 输出。DTMF 和 MSK 基带信号也由 DSP 生
成并由 IC507 输出。
LPF (IC701) 用作平滑滤波器。根据各自的调制方式对按
照发射载波输出的电平进行微调。
4-3. 驱动器和末级放大器
T/R 开关 (D17 开启 ) 的信号由驱动放大器 ( Q102) 放大
到 16 ～ 17dBm。驱动放大器的输出信号由末级放大器模块
(I C102) 放大到 25W ( 当功率低为 5.0W 时 )。末级放大器模
块的输出信号随即通过谐波滤波器 (LPF) 及天线开关 (D110、
D111 开启 ) 及方向性耦合器，然后加载到天线端子上。
4-4. APC 电路
通过使用方向性耦合器和二极管 ( D104 和 D105) 检测功
率模块输出，自动发射功率控制 (APC) 电路将发射器输出功
率稳定在预定电平。二极管 (D104 和 D105) 对直流放大器
IC103 (2/2) 施加电压。
配置 A P C 电路是为了保护因天线端负载波动而引起的功率
模块过流，以及在电压和温度变化时稳定发射输出。
NX-820
To TX
stage
21