Kenwood NX-800 Service Manual page 29

3-4. Audio amplifi er circuit
Audio processing (high-pass filter, low-pass filter, de-
emphasized and so on) at FM mode and decoding at NXDN
mode are processed by DSP. Audio signal from IC510,
IC502 goes through the amplifi er (IC701). The signal then
goes through mute switch (IC707), amplifier (IC711), and
electronic volume control (IC712).
While busy, INAMT becomes High, turn switch (Q709)
on and Mute switch (Q710) off, and signal is fed to AF Pow-
er Amp (IC715). While Non-busy, INAMT is become Low,
turn switch (Q709) off and Mute switch (Q710) on, then
there is not AF output.
3-5. Squelch Circuit
It amplifies the demodulated noise signal from IC202
after fi ltering through BPF circuit. Then, the amplifi ed signal
is converted to DC signal by the detection circuit. The con-
verted signal is fed back to IC510.
4. Transmitter System
4-1. Audio Band Circuit
The signal from the microphone goes through the mute
switch (Q706), the Mic-Mute signal (MM) becomes Low,
then mute switch (Q706) is turned off. The signal from
microphone goes through Mic-AGC (Q704, Q705, D704,
D705), and goes through switch IC (IC708), and amplifi ed by
Mic-Amp IC706 (1/2), LPF IC706 (2/2) works as anti-aliasing
fi lter.
If an optional scrambler board is installed, the switch
(IC708) adjusts the signal path so that the audio signal is in-
put to the scrambler board.
4-2. Base Band Circuit
The audio signal output from the base band circuit is con-
verted to digital data of a sampling frequency of 48kHz.
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 and an audio range 300Hz to 3kHz is extracted.
The audio signal is then pre-emphasized in FM mode and
synthesized with the signals, such as QT and DQT, as re-
quired, and is then output from the IC510. In Digital mode,
the audio signal is converted to the 4-Level FSK base band
signal and output from the IC510. The DTMF and MSK
base band signals are also generated by the DSP and output
by the IC510.
CIRCUIT DESCRIPTION /
IC705 (2/2) Q702
FMDET
IC202 BPF AMP
Fig. 4 / 图 4
电路说明
3-4. 音频放大器电路
FM 模式的音频处理 ( 高通滤波器、低通滤波器、去加重等 )
和 NXDN 模式的解码由 DSP 进行处理。IC510、IC502 的音频信
号通过放大器 ( I C701)。然后，信号通过静音开关 ( I C707)、
放大器 (IC711) 和电子音量控制 (IC712)。
繁忙时，I N A M T 变高，打开开关 ( Q709) 并关闭静音开关
( Q710)， 信 号 被 送 入 A F 功 率 放 大 器 ( I C715)。 不 繁 忙 时，
INAMT 变低，关闭开关 (Q709) 并打开静音开关 (Q710)，此时
没有 AF 输出。
3-5. 静噪电路
通过 BPF 电路滤波后， 该电路放大 IC202 的已解调噪音信号。
随后，放大的信号由检测电路转换为 D C 信号。转换的信号被
反馈给 IC510。
D703 IC705 (1/2)
ASQDET
RECT AMP IC510
IC203 (1/2)
VAGC
AMP
IC203 (2/2)
RSSI
AMP
4. 发射机系统
4-1. 音频频带电路
麦克风的信号通过静音开关 (Q706)，麦克风静音信号 (MM)
变低，然后静音开关 (Q706) 被关闭。麦克风的信号通过 Mic-
AGC(Q704、Q705、D704、D705)，然后通过开关 IC(IC708)，由
麦克风放大器 IC706(1/2) 放大，LPF IC706(2/2) 用作抗混叠
滤波器。
如果装有语音扰频器板，则开关 ( I C708) 将对信号路径进
行调整，使信号输入语音扰频器板。
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 C502)，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 3k H 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 Q T
和 D Q T 等信号合成，然后从 I C510 输出。在数字模式下，音
频信号被转换为 4 级 FSK 基带信号，从 IC510 输出。DTMF 和
MSK 基带信号也由 DSP 生成并由 IC510 输出。
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