Kenwood NX-411 Service Manual page 14

NX-411
4. Transmitter System
4-1. Audio Band Circuit
The signal from the internal microphone goes through
the mute switch (Q9). When the SP-MIC is not attached, the
microphone switching terminal (MSW) on the universal con-
nector becomes High, and the mute switch (Q9) is turned
on. When the SP-MIC is attached, MSW is connected to
GND inside of the SP-MIC. For this reason, Q8 is turned off,
the internal microphone is muted, and only the input of the
external microphone is supplied to the microphone ampli-
fi er. The signal from the microphone goes through the mute
switch (Q6), and is amplifi ed by IC14 and limited by the AGC
circuit which is composed of D1, D2, Q3 and Q4.
14
CIRCUIT DESCRIPTION
EXT. MIC
INT. MIC Q9 Q6
SW SW
MM
Q8
MSW
IC1
LPF
Fig. 5 Audio band and Baseband circuit
4-2. Baseband Circuit
The audio signal from the audio band circuit is converted
to digital data with a sampling frequency of 48kHz with ASIC
(IC309). This digital data is sent to the DSP (IC304), and
voice signals of 300Hz or lower and frequencies of 3kHz or
higher are cut off and an audio range of 300Hz to 3kHz is
extracted.
In FM mode, the audio signal is pre-emphasized and
combined with signals, such as QT and DQT, as required.
In Digital mode, the audio signal is converted to the
4-Level FSK baseband signal. The baseband signals of
DTMF and MSK are also generated by the DSP.
The processed baseband signal is output from IC304
and IC309. The signal from IC304 deviates the frequency of
the PLL IC (IC503). The signal from IC309 is fi ltered with the
LPF (IC1), and adjusts the level with DAC (IC3), and devi-
ates the frequency of the VCO.
IC14 IC8 IC5
AMP SW AMP
SCSW
AGC TCXO
Q3, Q4 X501
D1, D2
IC5 IC3 IC505
AMP DAC AMP
IC4 IC309
LPF ASIC
PLL DSP
IC503 IC304
VCO