Audio Amplifier Circuit; Squelch Circuit; Audio Band Circuit; Base Band Circuit - Kenwood NX-200 K2 Service Manual

3-3. Audio Amplifi er Circuit
Audio processing (high-pass filter, low-pass filter, de-
emphasized and so on) at FM mode and decoding at NXDN
mode are processed by DSP. The audio signal from IC108
and IC102 goes through the amplifier (IC704). The signal
then goes through a mute switch (IC707), amplifi er (IC713),
electronic volume control (IC703), and AF amplifi er (IC408).
While busy, AMT becomes Low to turn IC707 on, and
the signal is fed to the AF switch. While INAMT is High, the
AF switch (IC409) selects the internal speaker, and the au-
dio signal is fed to the internal audio power amplifi er (IC413),
and output to the internal speaker. While EXAMT is High,
the AF switch (IC409) selects the external speaker, and
the audio signal is fed to the external audio power ampli-
fi er (IC412), and output to the external speaker. The power
supply for IC413 and IC412 is turned on while INAMT or EX-
AMT is High.
The speaker is switched by the logic of the speaker
switching terminal SSW on the universal connector. When
the SP-MIC is not attached, SSW becomes High. IC108
detects the logic of SSW and activates either INAMT or EX-
AMT.
Q406
B
INAMT
EXAMT
AF signal
from IC408
Fig. 3 Audio amplifi er circuit
EXT. MIC
INT. MIC Q5 Q707
SW SW
MM
Q4
D703,D704
Q705,Q706
MSW
CIRCUIT DESCRIPTION
Q405
IC409
VDD
IC413 INT. SP
SHUT DOWN
VDD
IC412 EXT. SP
SHUT DOWN
IC716 IC710
(1/2) (1/2)
IC712 IC706
AMP SW AMP LPF
SCSW
AGC
Fig. 5 Audio band and Base band circuit

3-4. Squelch Circuit

It amplifies the demodulated noise signal from IC108
after filtering through the BPF circuit. Then, the amplified
signal is converted to a DC signal by the detection circuit.
The converted signal is fed back to IC108.
IC702
(1/2)
ASQAPC
DET LPF
ASQDET
IC108
Fig. 4 Squelch circuit
4. Transmitter System

4-1. Audio Band Circuit

The signal from the internal microphone goes through
the mute switch (Q5). When the SP-MIC is not attached, the
microphone switching terminal (MSW) on the universal con-
nector becomes High, and the mute switch (Q5) is turned
on. When the SP-MIC is attached, MSW is connected to
GND inside the SP-MIC. For this reason, Q5 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 (Q707), and is amplifi ed by IC716 (1/2) and limited by
the AGC circuit which is composed of D703, D704, Q705
and Q706.

4-2. Base Band Circuit

The audio signal output from the base band circuit is con-
verted to digital data with a sampling frequency of 48kHz.
This digital data is sent to the DSP (IC102), 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. The
audio signal is then pre-emphasized in FM mode and syn-
thesized with the signals, such as QT and DQT, as required,
and is then output from IC108. In Digital mode, the audio
signal is converted to the 4-Level FSK base band signal and
output from IC108. The DTMF and MSK base band signals
are also generated by the DSP and output by IC108.
LPF (IC705) works as a smoothing fi lter. The DAC (IC703)
assigns the base band signal to the VCO and VCTCXO (X1).
At this time, the level output according to the transmit car-
rier is fi ne-adjusted according to each modulation method.
IC102
DSP
IC705 IC705 IC710
(2/2) (1/2) (2/2)
IC108
LPF LPF AMP
ASIC
NX-200
IC711 IC711
(2/2) Q704 D705 (1/2)
HPF AMP RECT AMP
IC5 X1
IC703 AMP VCTCXO
AMP VCO
IC4
(1/2)
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