Audio Amplifier Circuit; Squelch Circuit; Transmitter System; Audio Band Circuit - Kenwood NX-210 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 IC308
and IC302 goes through the amplifier (IC802). The signal
then goes through a mute switch (IC805), amplifi er (IC809),
electronic volume control (IC801), and AF amplifi er (IC814).
While busy, AMT becomes Low to turn IC805 on, and
the signal is fed to the AF switch. While INAMT is High, the
AF switch (IC816) selects the internal speaker, and the au-
dio signal is fed to the internal audio power amplifi er (IC103),
and output to the internal speaker. While EXAMT is High,
the AF switch (IC816) selects the external speaker, and
the audio signal is fed to the external audio power ampli-
fi er (IC817), and output to the external speaker. The power
supply for IC103 and IC817 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. IC308
detects the logic of SSW and activates either INAMT or EX-
AMT.
Q810
B
INAMT
EXAMT
AF signal
from IC814
Fig. 3 Audio amplifi er circuit
EXT. MIC
INT. MIC Q812 Q807
SW SW
MM
Q811
D801,D802
Q803,Q804
MSW
CIRCUIT DESCRIPTION
Q806
IC816
VDD
IC103 INT. SP
SHUT DOWN
VDD
IC817 EXT. SP
SHUT DOWN
IC813 IC807
(1/2) (1/2)
IC808 IC804
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 IC308
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 IC308.
IC811
(1/2)
ASQAPC
DET LPF
ASQDET
IC308
Fig. 4 Squelch circuit

4. Transmitter System

4-1. Audio Band Circuit

The signal from the internal microphone goes through
the mute switch (Q812). When the SP-MIC is not attached,
the microphone switching terminal (MSW) on the universal
connector becomes High, and the mute switch (Q812) is
turned on. When the SP-MIC is attached, MSW is con-
nected to GND inside the SP-MIC. For this reason, Q812
is turned off, the internal microphone is muted, and only
the input of the external microphone is supplied to the mi-
crophone amplifier. The signal from the microphone goes
through the mute switch (Q807), and is amplifi ed by IC813
(1/2) and limited by the AGC circuit which is composed of
D801, D802, Q803 and Q804.
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 (IC302), 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 IC308. In Digital mode, the audio
signal is converted to the 4-Level FSK base band signal and
output from IC308. The DTMF and MSK base band signals
are also generated by the DSP and output by IC308.
LPF (IC803) works as a smoothing fi lter. The DAC (IC801)
assigns the base band signal to the VCO and VCTCXO (X501).
At this time, the level output according to the transmit car-
rier is fi ne-adjusted according to each modulation method.
IC302
DSP
IC803 IC803 IC807
(2/2) (1/2) (2/2)
IC308
LPF LPF AMP
ASIC
NX-210
IC815 IC815
(2/2) Q808 D807 (1/2)
HPF AMP RECT AMP
IC502 X501
IC801 AMP VCTCXO
AMP VCO
IC505
(1/2)
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