Transmitter Circuits - Icom IC-F4210D Service Manual

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.
• RX AF CIRCUIT
IC2
From the IF IC
IC58
AFVO
AF
AFO
AMP
Electronic
volume
AFON
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.
• SQUELCH CIRCUIT
NOISE SQUELCH DIAGRAM
To RX AF circuits
7
14 15
DAC
8
IC57
From IF IC
Noise filter
(IC3, Pin16)
IC55
J52
Ext.speaker
AF
AM P
J53
1
AF
2
AM P
SP1
Int.speaker
Noise
AMP 13
Com-
Noise
parator
detector
IC3

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.
• TX AF CIRCUITS
"NOIS"
To the modulation
circuit
[Ext. MIC]
J51
MC51
[Int. MIC]
4 - 2
Q901
W/N SW
DET
IC2
IC60
NJM13403V
R89
SQIN
BUFF
VOL1
BEEP
AFVO
BUFF
DAFO
REF
VCON
BUFF
BAL
VOL1
MOD
LPF
VOL1
LIMIT
HPF AMP
TX/RX
MOD
ALC
AMP LINE SW
AMP
CTRL
W/N SW
D53
VOXV
AMP DET
NWC
T1
R89 T2
LVA
VOL1
DISC
DMO
IC903
DSP
LINEAR
CODEC