Kenwood NX-220 Service Manual page 19

4. Transmitter System
4-1. Audio band circuit
The signal from the microphone is amplifi ed by IC2 (2/2)
and limited by the AGC circuit which is composed of D3,
D4, Q4 and Q5. IC2 works as an anti-aliasing LPF fi lter.
4-2. Baseband circuit
The audio signal output from the baseband circuit is con-
verted to digital data with a sampling frequency of 48kHz.
This digital data is sent to the DSP (IC603), and voice signals
of 300Hz or lower and frequencies of 3kHz or higher are cut
off and the audio range of 300Hz to 3kHz is extracted. The
audio signal is then pre-emphasized in FM mode and syn-
thesized with the signals, such as QT and DQT, as required,
and is then output from IC610. In Digital mode, the audio
signal is converted to the 4-Level FSK baseband signal and
output from IC610. The DTMF and MSK baseband signals
are also generated by the DSP and output by IC610.
The LPF (IC6) works as a smoothing fi lter. The level out-
put according to the transmit carrier is fi ne-adjusted accord-
ing to each modulation method.
Fig. 4 Audio band and Baseband circuit / 图 4 音频频带和基带电路
4-3. VOX
The DSP detects the audio level. If the detected level be-
comes higher than the threshold level, the VOX starts.
4-4. Drive and Final amplifi er
The signal from the T/R switch (D413 is on) is amplifi ed
by the pre-drive amplifier (Q102) to 16~17dBm. It makes
it possible to spoil input to the drive amplifier by control-
ling the power supply of the pre-drive amplifier with the
PD switch (Q110). The output of the pre-drive amplifi er is
amplifi ed by the drive and fi nal amplifi ers (Q103, Q104) to
5.0W (1.0W when the power is low). Q103 and Q104 are
MOS FETs. The output of the fi nal amplifi er is then passed
through the harmonic fi lter (LPF) and antenna switch (D102,
D103 are on) and applied to the antenna terminal.
4-5. APC circuit
The APC circuit always monitors the current flowing
through the RF power amplifi er (Q104) and keeps a constant
current. The voltage drop at R153, R154 and R155 is caused
CIRCUIT DESCRIPTION /
IC603
DSP
MIC IC2 (2/2)
AMP IC610
AGC
D3,D4
Q4,Q5
电路说明
4. 发射部系统
4-1. 音频频带电路
麦克风信号被 I C2(2/2) 放大并被 D3、D4、Q4 和 Q5 组成的
AGC 电路所限制。IC2 用作抗混叠 LPF 滤波器。
4-2. 基带电路
从基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 ( I C603)，300H z 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
3kHz 的音频范围。然后，音频信号以 FM 模式预加重，并与所
需的 QT 和 DQT 等信号合成， 然后从 IC610 输出。 在数字模式下，
音频信号被转换为 4 级 F S K 基带信号并从 I C610 输出。D T M F
和 MSK 基带信号也由 DSP 生成并由 IC610 输出。
LPF(IC6) 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。
IC403
PLL IC
IC6 (1/2)
LPF VCO to TX stage
4-3. VOX( 声控发射 )
D S P 检测音频电平。如果检测到的电平变得高于阈值电平，
VOX 启动。
4-4. 驱动器和末级放大器
T / R 开关 ( D413 开启 ) 的信号由预驱动放大器 ( Q102) 放大
到 16~17d B m。这有可能通过使用 P D 开关 ( Q110) 控制预驱
动放大器的电源而损坏输入到驱动放大器的信号。预驱动放
大器的输出信号由驱动器和末级放大器 ( Q103、104) 放大到
5.0W ( 当功率低为 1.0W 时 )。Q103 和 Q104 为 M O S F E T。末
级放大器的输出信号随即通过谐波滤波器 ( L P F ) 及天线开关
(D102、D103 开启 )，然后加载到天线端子上。
4-5. APC 电路
A P C 电路随时监测通过 R F 功率放大器 ( Q104) 的电流，并
保持恒定电流。R153、R154 及 R155 的压降是由通过 R F 功率
放大器的电流所形成的， 并将此电压加载到差分放大器 (IC101
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