Kenwood TK-3148 Service Manual page 13

3-5. Squelch circuit
The output from IC200 enters FM IF IC again, then passed
through a band-pass filter. And passed through a band-pass
filter IC202 (2/2). The modulation input. The noise component
output from IC202 (2/2) is amplified by Q208 and rectified by
D201 to produce a DC voltage corresponding to the noise level.
The DC voltage is sent to the analog port of the CPU (IC309).
And IC200 outputs a DC voltage (RSSI) corresponding to the
input of the IF amplifier. The CPU reads the RSSI signal via pin
93.
IC309 determines whether to output sounds from the
speaker by comparing the input voltage of pin 91 and pin 93
with the preset value.
4. Transmitter System
4-1. Microphone amplifier
The signal from the internal microphone goes through the
mute switch (Q301).
When the SP-MIC is not attached, the microphone
switching terminal (MSW) on the universal connector becomes
High, and mute switch (Q301) is turned ON. When the SP-
MIC is attached, MSW is connected to GND at inside of SP-
MIC. For this reason, Q301 is turned OFF, the internal
microphone is muted, and only the input of the external
microphone is supplied to the microphone amplifier of the TX-
RX unit.
The signal from microphone passes through the limited
circuit in D508 and amplified by IC501 (1/2), Microphone mute
switch (Q502 is off in TX) and through the high-pass filter, the
ALC circuit, the low-pass filter, the high-pass filter, and pre-
emphasis/ IDC circuit in IC500 on the small board. When
encoding DTMF, mute switch (Q500) is turned OFF for muting
the microphone input signal.
The signal passes through the D/A converter (IC307) for
the maximum deviation adjustment, and enters the summing
amplifier consisting of IC305 (1/2), and is mixed with the low
speed data from the CPU (IC309).
CIRCUIT DESCRIPTION /
IC200 : FM IF IC
IF AMP
DET
RSSI BPF
AMP
7
12
SQ close
SQ open
ANT input level
Fig. 6 Squelch and RSSI voltoge vs ANT input level /
图 6 静噪和 RSSI 电压与 ANT 输入电平
电路说明
3-5．静噪电路
IC200 的输出再次进入调频中频芯片，然后通过片上的带通
滤波器。并通过带通滤波器 IC202（2/2） 。调制输入。IC202（2/
2） 输出的噪音成分被 Q208 放大并且被 D201 整流生成一个相对
应于噪音电平的直流电压。直流电压被发送到 CPU（IC309）的
模拟端口。 IC200生成一个相对应于中频放大器输入的直流电压
（RSSI） 。CPU 通过管脚 93 读取 RSSI。
IC309将管脚91和管脚93的输入电压与预设值进行比较后决
定是否通过扬声器输出声音。
91
IC309
CPU
IC202
(2/2) Q208 D201
BPF NOISE
DET
AMP AMP
93
Preset
value
Preset balue
ANT input level
4．发射部系统
4-1．话筒放大器
来自于内置话筒的信号进入静音开关（Q301） 。
当扬声器 - 话筒不接上时，通用接口上的话筒开关端点
（MSW）变为高电平，静音开关（Q301）接通。当扬声器 - 话筒
接上时， MSW 与扬声器 - 话筒内的地线连接。 因此， Q301 断开，
内置话筒静音， 并且只有外置话筒的输入提供给 TX-RX 单元的
话筒放大器。
来自麦克风的信号通过 D508 中的限幅电路，经过 IC501 (1/
2) 放大、麦克风静音开关（发射时 Q502 关断） ，通过高通滤波
器，ALC 电路，低通滤波器，高通滤波器，以及小板 IC500 上
的预加重 /IDC 电路。 当编码 DTMF 时， 静音开关 （Q500） 关闭，
以阻断麦克风输入信号。
信号通过数 / 模转换器（IC307）进行最大频偏调整，然后进
入由IC305 （1/2） 组成的总和放大器， 并且与来自于CPU （IC309）
的低速率数据混合。
TK-3148
13