4 - 2
• RX AF CIRCUIT
• SQUELCH CIRCUIT
• TX AF CIRCUITS
4-2 TRANSMITTER CIRCUITS
TX AF CIRCUITS
The audio signal from the internal or external microphone
(MIC signal) is applied to IC2 (pin 13). The MIC signal is
amplifi ed by the MIC AMP (inside IC2) and then output from
pin 11.
• WHILE OPERATING IN THE ANALOG MODE
The MIC signal is passed through the HPF (IC2, pin 1), which
attenuates frequencies 300 Hz and below, and then applied
to the limiter AMP (pin 4 of IC2). The amplitude-limited MIC
signal is applied to IC2.
The MIC signal is converted into a digital audio signal by the
linear codec (inside IC2), processed by the DSP (IC903),
and then converted into an analog baseband signal (modu-
lation signal).
• WHILE OPERATING IN THE DIGITAL MODE
The MIC signal is applied to the ALC (IC2, pin10) which
keeps the signal level fi xed.
The level-adjusted MIC signal is applied to the linear codec
(inside IC2) through the MIC line SW (inside IC2).
The MIC signal is converted into a digital audio signal by the
linear codec (inside IC2), processed by the DSP (IC903),
and then converted into the digital baseband signal (modula-
tion signal).
The signal from the linear codec (inside IC2) is passed
through the LPF (inside IC2), and then applied to the D/A
converter (inside IC2) which adjusts its level (=deviation).
The level-adjusted modulation signal is applied to the modu-
lation circuit.
AF
AM P
AF
AMP
IC55
J52
SP1
AF
AM P
J53
Int.speaker
Ext.speaker
1
2
IC58
AFON
AFO
From the IF IC
AFVO
Electronic
volume
IC2
DAC
Noise
AMP
Noise filter
From IF IC
(IC3, Pin16)
To RX AF circuits
Noise
detector
Com-
parator
NOISE SQUELCH DIAGRAM
“NOIS”
IC57
IC3
14
8
7
13
15
VOL1
R89
AMP
ALC
AMP
VOL1
AMP
LPF
BUFF
MC51
J51
VOL1
W/N SW
Q901
BUFF
LIMIT
AMP
DET
D53
W/N SW
BUFF
IC60
NJM13403V
HPF
MOD
LINE
CTRL
VOL1
R89
DSP
IC903
LINEAR
REF
DET
AFVO
BEEP
MOD
TX/RX
SW
DMO
IC2
BAL
CODEC
DAFO
SQIN
VCON
DISC
VOXV
T1
LVA
T2
NWC
[Ext. MIC]
[Int. MIC]
To the modulation
circuit
SQUELCH CIRCUIT (Analog mode only)
The squelch circuit cuts off the AF output signals when no
RF signals are received. Detecting noise components in the
demodulated AF signals, the squelch circuit stops audio sig-
nals being heard.
A portion of the demodulated AF signal from the IF IC (IC3)
is passed through IC2 for level (=threshold) adjustment. The
level-adjusted AF signals are passed through the noise fi lter
(IC3, pins 7, 8 and R139–R142, C241–C243) to filter only
the noise components (approximately 30 kHz signals). The
noise components are rectified, resulting in a DC voltage
corresponding to the noise level.
If the noise level is higher than the preset one, the internal
comparator sets the “NOISE” signal to the CPU to “High,”
then the CPU turns the “AFON” signal which controls the AF
power AMP (IC55) to “Low,” to deactivate it.
RX AF CIRCUITS
The detected audio signals adjusted by the linear codec’s
electronic volume (inside IC2). The level-adjusted AF signal
is then amplifi ed by the pre-AMP (IC58) and AF power AMP
(IC55).
The amplifi ed AF signal is applied to the internal or external
speaker.