TK-3178
(
L
)
13
2-3. IF Amplifi er Circuit
The fi rst IF signal is passed through a four-pole monolithic
crystal fi lter (XF701) to remove the adjacent channel signal.
The fi ltered fi rst IF signal is amplifi ed by the fi rst IF amplifi er
(Q703) and then applied to the lF system IC (IC701). The IF
system IC provides a second mixer, second local oscillator,
limiting amplifi er, quadrature detector and RSSI (Received
Signal Strength Indicator). The second mixer mixes the fi rst
IF signal with the 50.4MHz of the second local oscillator
output (TCXO X501) and produces the second IF signal of
450kHz.
The second IF signal is passed through the ceramic fi lter
(Wide: CF701, Narrow: CF702) to remove the adjacent chan-
nel signal. The fi ltered second IF signal is amplifi ed by the
limiting amplifi er and demodulated by the quadrature detec-
tor with the ceramic discriminator (CD701). The demodu-
lated signal is routed to the audio circuit.
2-4. Wide/Narrow Switching Circuit
Wide and Narrow settings can be made for each channel
by switching the ceramic fi lters CF701 (Wide), CF702 (Nar-
row). The Wide and Narrow is output from IC4.
D701 and D702 are switched to ceramic fi lters when a
Wide/Narrow level is selected.
Q702 turns on/off with the Narrow and the IC701 detec-
tor output level is changed to maintain a constant output
level during wide or narrow signals.
2-3. 中频放大电路
第一中频信号通过晶体滤波器 ( X F701) 消除相邻信道的信
号。经滤波的第一中频信号被第一中频放大器 ( Q703) 放大并
进入中频系统芯片 ( I C701)。中频系统芯片提供第二混频器、
第二本振信号、限幅放大器、正交检测器和 R S S I ( 接收信号
强度指示器 )。第二混频器将第一中频信号与 50.4MHz 的第二
本振信号输出 (TCXO X501) 进行混频,并生成 450kHz 的第二
中频信号。
第二中频信号通过陶瓷滤波器 ( 宽 : C F701、窄 : C F702) 继
续消除相邻信道的信号。经滤波的第二中频信号被限幅放大
器放大并被带有陶瓷鉴频器 ( C D701) 的正交检测器解调。经
解调的信号进入音频电路。
2-4. 宽 / 窄切换电路
通过切换陶瓷滤波器 CF701( 宽 )、CF702( 窄 ) 可以对每一
信道进行宽,窄设置。宽、窄控制信号从 IC4 输出。
选择宽 / 窄电平时,将 D701 和 D702 切换到陶瓷滤波器。
在宽或窄信号期间,Q702 随窄信号打开 / 关闭,I C701 检
测器输出电平变为保持恒定输出电平。
2-5. Audio Amplifi er Circuit
The demodulated signal from IC701 is amplifi ed by IC10,
and goes to AF amplifi er through IC13.
The signal then goes through an volume control (IC16),
and is routed to an audio power amplifi er (IC18) where it is
amplifi ed and output to the speaker.
2-5. 音频放大器电路
来自于 I C701 的解调信号被 I C10 放大,并通过 I C13 送到
AF 放大器。
信号通过 A F 音量控制 ( I C16),在音频功率放大器 ( I C18)
进行放大后输出到扬声器。
MXO
AFOUT
QUAD
IFOUT
IFI
Wide
C702
C701
C708
C719
R705
R702
R701
R703
R706
C709
R708
D702
D701
CF701
CF702
IC701
R719
R717
Q702
R718
5C
AFOUT
Narrow
Narrow
CD701
Fig. 3 Wide/Narrow switching circuit /
图 3 宽 / 窄切换电路
CIRCUIT DESCRIPTION /
电路说明