Transmitter Circuits - Icom IC-F4261DT Service Manual

RX AF CIRCUIT (MAIN AND FRONT UNITS)
The AF signal from the demodulator circuit is applied to the
baseband IC (MAIN UNIT: IC4, pin 23).
The applied AF signal is amplified and adjusted in level,
and then unwanted 3 kHz and higher audio signals are sup-
pressed by the LPF.
The filtered AF signal is sequentially passed through (or
bypassed) the TX/RX HPF, scrambler and de-emphasis cir-
cuits, depending on the transceiver's setting.
The processed AF signal is output from the base band IC
(MAIN UNIT: IC4, pin 20), and passed through the AF mute
SW (MAIN UNIT: IC8, pins 3 and 4) and adjusted in level by
the D/A converter (MAIN UNIT: IC9, pins 15 and 16), and
then applied to the FRONT UNIT.
The AF signal is sequentially amplified by the AF AMP
(FRONT UNIT: IC705) and AF power AMP (FRONT UNIT:
IC706) to obtain an AF output power level.
The amplified AF signal is applied to the internal speaker
(CHASSIS: SP1).
When an external speaker is connected to the 14-pin jack,
the AF signal is applied to the external speaker, after being
amplifi ed by the AF power AMP (FRONT UNIT: IC706).
• RX AF CIRCUITS
MAIN-A/B/C UNIT
IC8
IC9
J5 IC4
RXAF
AF
Baseband IC
DA6 MUTE
J701
IC705 IC706
AF
AF
AF
AMP AMP
FRONT UNIT
IC8
From the demodulator circuit
Digi/
Ana
IC706
SP1
AF
AMP
MP801
AFO
14PIN
JACK UNIT
4-2 TRANSMIT CIRCUITS
TX AF CIRCUIT (MAIN AND FRONT UNITS)
The AF signal from the internal or external microphone
(MIC signal) is passed through the MIC SW (FRONT UNIT:
Q708), and applied to the MIC AMP (FRONT UNIT: IC705,
pin 6). The amplifi ed MIC signal is applied to the MAIN UNIT.
• WHILE OPERATING IN THE ANALOG MODE
The MIC signal from the FRONT UNIT is amplified by the
AF AMP (MAIN UNIT: IC3), and passed through the AF SW
(MAIN UNIT: IC7, pins 1 and 7), and then applied to the
baseband IC (MAIN UNIT: IC4, pin 3).
The applied MIC signal is amplified and adjusted in level.
The level-adjusted MIC signal is sequentially passed through
(or bypasses) the compressor, pre-emphasis circuit, TX/
RX HPF, de-scrambler, limiter, splatter circuit which cuts off
the signals whose frequency is higher than voice frequency,
depending on the transceiver's setting, and then output from
pin 7.
The processed AF signal is passed through the mode SW
(MAIN UNIT: IC8, pins 12 and 14), PM/FM SW (MAIN UNIT:
IC6, pins 12 and 14), and then applied to the modulation cir-
cuit.
• WHILE OPERATING IN THE DIGITAL MODE
The MIC signal from the FRONT UNIT is amplified by the
AF AMP (MAIN UNIT: IC3), and passed through the AF SW
(MAIN UNIT: IC7, pins 1, 6), and then further amplified by
the ALC AMP (MAIN UNIT: IC1).
The amplified signal is applied to the baseband IC (MAIN
UNIT: IC4, pin 3), through the AF SW (MAIN UNIT: Q3).
The applied MIC signal is amplifi ed by the internal AMP and
limit AMP which limits the amplitude, and then is output from
pin 7, through the LPF.
The processed AF signal is amplified by the buffer (MAIN
UNIT: IC12), and then applied to the liner CODEC (MAIN
UNIT: IC23, pin 2), to be encoded into a digital audio signal.
The digital audio signal is processed by the DSP (MAIN
UNIT: IC24), and then applied to the linear codec (MAIN
UNIT: IC23) to be converted into the digital baseband signal.
The digital baseband signal is amplifi ed by the AMP (MAIN
UNIT: IC18), and then applied to the modulation circuit,
through the mode SW (MAIN UNIT: IC17, pins 1 and 2).
• TX AF CIRCUITS
MAIN-A/B/C UNIT
J5
IC3 IC7
MCIN
AMP
To the modulation circuit
FRONT UNIT
J701
IC705
MCIN
MIC
AMP
4 - 2
IC6
PM/FM
IC1
Q3
SW
ALC
Digi/
Digi/
AMP
Ana
Ana
IC9
IC3
DA2 AMP
MODA
MP801
14PIN
Q710
JACK UNIT
Q709
Q708
SW
D705
MIC
MC701
IC4
Baseband IC
IC8
Digi/
Ana
MICA