Transmitter Circuits - Icom IC-F29DR Service Manual

4-2 TRANSMITTER CIRCUITS

TX AF CIRCUITS (MAIN UNIT)
The audio signal from the internal or external microphone
(MIC signal) is applied to the AF custom IC (IC5).
• WHILE OPERATING IN THE ANALOG MODE
The MIC signal is amplifi ed and passed through the HPF, and
then applied to the limiter AMP. The amplitude-limited signal
is applied to the linear codec, through the MIC line SW.
The MIC signal is converted into a digital audio signal by the
linear codec, processed by the DSP (IC15), and then con-
verted into an analog baseband signal (modulation signal).
• WHILE OPERATING IN THE DIGITAL MODE
The MIC signal is amplified and applied to the ALC AMP,
which keeps the signal level fi xed.
The MIC signal is converted into a digital audio signal by the
linear codec, processed by the DSP (IC15), and then con-
verted into the digital baseband signal (modulation signal).
The signal from the linear codec (inside the AF custom IC
(IC5)) is passed through the LPF and the D/A converter,
which adjusts the signal in level (=deviation). The level-
adjusted modulation signal is output from the AF custom IC
(IC5), and then applied to the modulation circuit.
• TX AF CIRCUITS
IC5 AF CUSTOM IC
AFVOL
AFO
BUFF
REF
VCON
BUFF
BAL
MOD
J2
ALC
EXT_MIC
AMP
MC1
MIC
AMP HPF
INT_MIC
MODULATION CIRCUIT (MAIN UNIT)
The modulation signal from the TX AF circuits is applied
to D9 of the TX VCO (Q5, D6, D8 and D9) to modulate it
(FM for the analog mode, 4FSK for the digital mode). The
modulated signal from the TX VCO is buffer-amplifi ed by two
buffers (Q11 and Q16) and applied to the TX AMP circuits,
through the LO SW (D12).
• MODULATION CIRCUIT
Q5,
D8
D6,9
FM
MOD
TX_VCO
MOD
Q6,
D7,10
RX VCO
BEEP
BUFF
MOD
DMO
LPF
MOD
TX/RX
LINE
SW
CTRL
LIMIT
AMP
Q11
Q16 D12,13
RF TX/RX
BUFF
AMP SW
To the 1st mixer
TX AMPLIFIERS
The buffer amplified signal from the LO SW (D12) is se-
quentially amplifi ed by the pre-drive AMP (Q18), drive AMP
(Q21), and power AMP (Q23), to obtain TX power. The am-
plified TX signal is passed through the antenna SW (D18)
and the LPF (L29, L32, L34, C191, C194, C199, C202 and
C205), which eliminates harmonics, and then fed to the an-
tenna.
APC CIRCUITS (PA UNIT)
D24 and D25 rectify a portion of the TX signal to direct cur-
rent, and the APC AMP (IC3) compares the voltage and the
TX power control reference voltage, "RFPC." The resulting
voltage controls the gain of the power and drive AMPs to
keep the TX power constant.
• TX AMP AND APC CIRCUITS
Q18
Q21
PRE
DRIVE
DRIVE
AMP
DISC
IC15
DSP
4-2
Q23
D18,23
PWR ANT
AMP SW
D24,25
IC3
APC
RFPC
AMP
To the RX circuit
LPF
PWR
DET