TK-3360
11
Fig. 3 Wide/Narrow switching circuit /
图 3 宽 / 窄切换电路
CIRCUIT DESCRIPTION /
电路说明
2-2. First Mixer
The signal from the front end is mixed with the fi rst local
oscillator signal generated in the PLL circuit by Q406 to pro-
duce a fi rst IF frequency of 49.95 MHz.
The resulting signal passes through the XF401 MCF to
cut the adjacent spurious and provide the optimum charac-
teristics, such as adjacent frequency selectivity.
2-3. IF Amplifi er Circuit
The fi rst IF signal is passed through a four-pole monolithic
crystal fi lter (XF401) to remove the adjacent channel signal.
The fi ltered fi rst IF signal is amplifi ed by the fi rst IF amplifi er
(Q405) and then applied to the lF system IC (IC401).
The IF system IC provides a second mixer, second local
oscillator, limiting amplifier, quadrature detector and RSSI
(Received Signal Strength Indicator). The second mixer
mixes the fi rst IF signal with the 50.4MHz of the second lo-
cal oscillator output (TCXO X1) and produces the second IF
signal of 450kHz.
The second IF signal is passed through the ceramic fi lter
(Wide: CF402, Narrow: CF401) 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 (CD401). 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 CF402 (Wide), CF401 (Nar-
row). The Wide and Narrow switching data is output from
IC809.
D401 and D402 are switched to ceramic fi lters when a
Wide/Narrow level is selected.
AFOUT
50C
IC401
R426
Narrow
D401
CF402
IFOUT
MXO
IFI
C424
QUAD
AFOUT
Wide
R424
CD401
C413
R406
C407
C402
C414
R409
R412
R408
R411
R415
D402
CF401
2-2. 第一混频器
前端的信号与 P L L 电路产生的第一本振信号在 Q406 混频，
生成 49.95MHz 频率的第一中频信号。
生成的信号通过 XF401 MCF。
2-3. 中频放大电路
第一中频信号通过晶体滤波器 ( X F401) 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 ( Q405) 放大并
进入中频系统芯片 (IC401)。
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 RSSI( 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 X1) 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 F402、窄 : C F401) 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 D401) 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 CF402( 宽 )、CF401( 窄 ) 可以对每一
信道进行宽，窄设置。宽、窄控制信号从 IC809 输出。
选择宽 / 窄电平时，将 D401 和 D402 切换到陶瓷滤波器。
