Kenwood NX-300 Service Manual page 34

NX-300
4. Transmitter System
4-1. Audio Band Circuit
The signal from the internal microphone goes through
the mute switch (Q5). When the SP-MIC is not attached, the
microphone switching terminal (MSW) on the universal con-
nector becomes High, and the mute switch (Q5) is turned
on. When the SP-MIC is attached, MSW is connected to
GND inside the SP-MIC. For this reason, Q5 is turned off,
the internal microphone is muted, and only the input of the
external microphone is supplied to the microphone ampli-
fi er. The signal from the microphone goes through the mute
switch (Q707), and is amplifi ed by IC716 (1/2) and limited by
the AGC circuit which is composed of D703, D704, Q705
and Q706.
4-2. Base Band Circuit
The audio signal output from the base band circuit is con-
verted to digital data with a sampling frequency of 48kHz.
This digital data is sent to the DSP (IC102), and voice signals
of 300Hz or lower and frequencies of 3kHz or higher are cut
off and an 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 IC108. In Digital mode, the audio
signal is converted to the 4-Level FSK base band signal and
output from IC108. The DTMF and MSK base band signals
are also generated by the DSP and output by IC108.
LPF (IC705) works as a smoothing fi lter. The DAC (IC703)
assigns the base band signal to the VCO and VCTCXO (X500).
At this time, the level output according to the transmit car-
rier is fi ne-adjusted according to each modulation method.
EXT. MIC
INT. MIC Q5
SW
Q4
MSW
4-3. VOX
IC716 (2/2) amplifi es the audio signal captured in the mi-
crophone. The signal is then converted into the DC voltage,
rectifi ed by D706. The DC voltage activates the ASIC (IC108),
and the VOX starts.
34
CIRCUIT DESCRIPTION /
IC716 IC710
(1/2) (1/2)
Q707 IC712
SW AMP SW AMP
MM SCSW
AGC
D703,D704
Q705,Q706
Fig. 5 Audio band and Base band circuit / 图 5 音频频带和基带电路
电路说明
4. 发射部系统
4-1. 音频频带电路
内部麦克风的信号通过静音开关 ( Q5)。如未连接 S P - M I C，
则通用连接器上的麦克风切换端子 ( M S W ) 变为高，同时静音
开关 (Q5) 打开。如连接 SP-MIC，MSW 在 SP-MIC 内部与 GND
连接。因此，Q5 关闭，内部麦克风静音，仅外部麦克风的输
入被提供到麦克风放大器。
麦克风的信号通过静音开关 ( Q707)，由 I C716(1/2) 进行
放大，并由 A G C 电路限幅。A G C 电路由 D703，D704，Q705 和
Q706 组成。
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
数字信号。该数字信号被发送给 D S P ( I C102)，300H z 或更低
的语音信号和 3k H z 或更高的频率被截止，并选取 300H z 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 IC108 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，从 I C108 输出。D T M F
和 MSK 基带信号也由 DSP 生成并由 IC108 输出。
LPF(IC705) 用作平滑滤波器。DAC(IC703) 将基带信号分配
到 VCO 和 VCTCXO(X500)。此时，将按照各自的调制方式对按
照发射载波输出的电平进行微调。
IC102
DSP
IC705 IC705
(2/2) (1/2)
IC706
IC108
LPF LPF LPF
ASIC
4-3. VOX( 声控发射 )
IC716(2/2) 放大麦克风中采集的音频信号。
然后，信号由 D706 进行整流，转换为 D C 电压。D C 电压激
活 ASIC(IC108)，VOX 启动。
IC710
(2/2)
IC501
AMP IC703 AMP VCTCXO
AMP
IC504
X500
VCO