Kenwood NX-300 Service Manual page 32

NX-300
3. Receiver System
3-1. RF Circuit
An incoming RF signal from the antenna terminal is
passed through the antenna switch (D606, D607, D709,
D711) and then the bandpass fi lter (L721, L722). The band-
pass fi lter is adjusted by a variable capacitor. The input volt-
age to the variable capacitor is regulated by the voltage out-
put from the D/A converter (IC703). The signal is amplifi ed
by an RF amplifi er (Q705), and passed through the bandpass
fi lter (L713, L714, L715). The resulting signal is applied to
the fi rst mixer (Q703), where it is mixed with the fi rst local
oscillator signal output from the frequency synthesizer to
produce the fi rst IF (58.05MHz).
3-2. IF Circuit
The fi rst IF signal is passed through a four-pole mono-
lithic crystal fi lter (XF700) to reject adjacent channel signals.
The fi ltered fi rst IF signal is amplifi ed by the fi rst IF amplifi er
(Q701) and then applied to the lF system IC (IC701). The IF
system IC provides a second mixer, AGC amplifi er, and RSSI
(Received Signal Strength Indicator).
The second mixer mixes the first IF signal with the
57.6MHz of second local oscillator output and produces the
second IF signal of 450kHz.
The second IF signal is passed through the ceramic fi lter
(CF700) to reject the adjacent channel signal. The filtered
second IF signal is amplifi ed by the AGC amplifi er.
The signal from the AGC amplifi er is input to the ASIC
(IC108) through the ceramic fi lter (CF701) and operational
amplifi er (IC700).
ANT
L612~L614
LPF
D606,D607
D709,D711
IC108
ASIC
IC102
DSP
32
CIRCUIT DESCRIPTION /
RF AMP
Q705
L721,L722 L713~L715
ANT
BPF BPF
SW
1st PLL OSC (PLL)
IC704 IC707
SW
IC713
(2/2)
AMT
Fig. 2 RF and IF circuit / 图 2 RF 和 IF 电路
电路说明
3. 接收部系统
3-1. RF 电路
天线端子接收到的 RF 信号依次通过天线开关 (D606，D607，
D709 和 D711) 和带通滤波器 ( L721，L722)。带通滤波器经
由可变电容器调整。可变电容器的输入电压经由数模转换器
( I C703) 的输出电压调整。随后，信号被 R F 放大器 ( Q705)
放大，然后通过带通滤波器 ( L713，L714，L715)。最终的信
号被运用到第一混频器 ( Q703)。在该混频器中，信号与频率
合成器输出的第一本地振荡器信号混频，从而产生第一中频
(58.05MHz)。
3-2. 中频电路
第一中频信号经过一个四极的单片晶体滤波器 ( X F700) 以
消除临近信道的信号。
滤波后的第一中频信号经过第一中频放大器 ( Q701) 的放大
后，加到中频系统 IC(IC701) 上。
中频系统 IC 提供一个第二混频器，AGC 放大器和 RSSI( 接
收信号强度指示器 )。
第二混频器混合第一中频信号及 57.6M H 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 ( C F700) 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。
A G C 放大器的信号通过陶瓷滤波器 ( C F701) 和运算放大器
(IC700) 输入 ASIC(IC108)。
MCF IF AMP
1st MIX
XF700 Q701
Q703
BPF
2nd OSC
X500
VC
x3
TCXO
Q700
19.2MHz
IC703 IC408 IC409
VOL SW
CF701
IC701
BPF
MIX, AGC
BPF
CF700
IC413
INT. SP
IC412
EXT. SP
IC700