电路说明 - Kenwood NX-720 Service Manual

NX-720
The second mixer mixes the first IF signal with the
50.4MHz of the second local oscillator output and produces
the second IF signal of 450kHz.
The second IF signal is passed through the ceramic fi lter
(CF1) to reject the adjacent channel signal. The fi ltered sec-
ond IF signal is amplifi ed by the AGC amplifi er.
The signal from the AGC amplifi er is input to the ASIC
(IC507) through the ceramic fi lter (CF2).
ANT
L116,L117 D110,D111
LPF
3-3. Audio amplifi er circuit
Audio processing (high-pass filter, low-pass filter, de-
emphasized and so on) in FM mode and decoding in NXDN
mode are processed by the DSP (IC502). Audio signals from
the ASIC (IC507), IC502 goes through the amplifi er (IC711).
The signal then goes through the D/A converter (IC712) and
an amplifi er (IC714).
3-4. Squelch Circuit
It amplifies the demodulated noise signal from ASIC
(IC507) after fi ltering through the LPF and HPF circuit. The
amplifi ed signal is then converted to a DC signal by the de-
tection circuit. The converted signal is fed back to IC507.
20
CIRCUIT DESCRIPTION / 电路说明
D107,
Q202
L215,L216
RF AMP
ANT
BPF
SW
IC 711
(1/2)
IC 507
ASIC
IC 502
DSP
Receiver System / 图 2 接收机系统
Fig.2
ASQAPC
DET
ASQDET
IC507
Fig. 3 Squelch Circuit / 图 3 静噪电路
第二混频器混合第一中频信号及 50.4MH z 的第二本地振荡
器输出，生成 450kHz 的第二中频信号。
第二中频信号通过陶瓷滤波器 (CF1) 以消除临近信道的信
号。滤波后的信号由 AGC 放大器进行放大。
AGC 放大器的信号通过陶瓷滤波器 (CF2) 被输入 AS IC
(IC507)。
L207,L209 Q201
XF1
Q305
1st MIX
L210,L220
MCF IF AMP
BPF
X1
1st local
OSC (VCO/PLL)
TCXO
16.8MHz
IC 711 IC 712
IC 714
(2/2)
VOL
3-3. 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 (IC502) 进行处理。ASIC (IC507)
和 IC502 的音频信号通过放大器 ( IC711)。然后信号通过数
模转换器 (IC712) 和放大器 (IC714)。
3-4. 静噪电路
通过 LPF 和 HPF 电路滤波后，该电路放大 ASIC (IC507)
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 IC507。
Q702
IC705(1/2)
D702
AMP
RECT
IC303
MIX,AGC
Q303
CF1
x3
SP
IC705(2/2)
ASQ
AMP
CF2
BPF