Kenwood NX-300 Service Manual page 14

NX-300(G)
3-3. Audio Amplifier 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 IC719 goes through the amplifier (IC704). The signal
then goes through a mute switch (IC707), amplifier (IC713),
electronic volume control (IC703), and AF amplifier (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 audio
signal is fed to the internal audio power amplifier (IC413),
and output to the internal speaker. While EXAMT is High, the
AF switch (IC409) selects the external speaker, and the au-
dio signal is fed to the external audio power amplifier (IC412),
and output to the external speaker. The power supply for
IC413 and IC412 is turned on while INAMT or EXAMT 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.
B
INAMT
EXAMT
AF signal
from IC408
Fig. 3 Audio amplifier circuit
EXT. MIC
INT. MIC Q5 Q707
SW SW
MM
Q4
MSW
14
CIRCUIT DESCRIPTION
Q406
Q405
IC409
VDD
IC413 INT. SP
SHUT DOWN
VDD
IC412 EXT. SP
SHUT DOWN
IC716 IC710
(1/2) (1/2)
IC712
AMP SW AMP
SCSW
AGC
D703,D704
Q705,Q706
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 sig-
nal is converted to a DC signal by the detection circuit. The
converted signal is fed back to IC108.
ASQAPC
DET
ASQDET
IC108
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-
fier. The signal from the microphone goes through the mute
switch (Q707), and is amplified 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 (IC719), and voice sig-
nals 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
synthesized with the signals, such as QT and DQT, as re-
quired, and is then output from IC108. In Digital mode, the
audio signal is converted to the 4-Level FSK base band sig-
nal 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 filter. The DAC (IC703)
assigns the base band signal to the VCO. At this time, the
level output according to the transmit carrier is fine-adjusted
according to each modulation method.
IC719
DSP
IC705 IC705
(2/2) (1/2)
IC706
IC108
LPF LPF LPF
ASIC
IC702 IC711
(1/2) (2/2) Q704
LPF HPF AMP
Fig. 4 Squelch circuit
IC710
(2/2)
IC502
AMP IC703 PLL
AMP
IC504
IC711
D705 (1/2)
RECT AMP
X500
VCTCXO
VCO