Transmitter Circuits - Icom IC-M35 Service Manual

4-2 TRANSMITTER CIRCUITS

TX AF CIRCUITS
AF signals from the microphone (MIC signals) are passed
through the MIC line SW (IC260A) and amplified by MIC
AMP (IC200B), then passed through the MIC mute SW
(IC260C).
The MIC line SW (IC260A) selects the MIC signal path (from
int. MIC or from ext. MIC), and the MIC mute SW (IC260C)
disconnects the MIC line in receiving.
The MIC signals from the MIC mute SW (IC260C) are
adjusted its amplitude (=deviation) by the D/A converter
(IC190), and pre-emphasized by R201 and C385 to obtain
+6 dB/oct of frequency response.
Pre-emphasized MIC signals are amplitude-limited by the
limiter (IC200A) to prevent over deviation, and fi ltered by the
splatter fi lter (IC200D) to remove 3 kHz and higher signals,
then applied to the VCO for frequency modulation.
• TX AF AND MODULATION CIRCUITS
Q 21,Q22
D22,D23
MOD
MUT E
MOD
MIC
[SP MIC]
IC200D
J250
LPF
MC250
Int. MIC
EXTMIC
S250
[PTT]
IC260A
INMIC
MIC LINE
SW
Q23 Q24
To LO switch
BUFF BUFF
Q20
D 20
D21
VCO
IC190
D/A
IC200A
PRE
LIMIT
AM P ENPHA
IC200B
IC260C
MIC MIC
AMP MUT E
TX AMPLIFIERS
The frequency-modulated signal from the VCO (TX signal)
is passed through the LO SW (D50), and amplified by the
buffer AMP (Q50), driver AMP (Q53) and power AMP (Q54)
in sequence, to obtain TX output power.
APC CIRCUIT
A portion of TX output signal is rectified by D91 to be
converted into the DC voltage which corresponds to TX
power level.
IC50 compares the detected voltage and TX power setting
voltage from D/A converter (IC190). The resulting voltage
from IC50 controls the gain of both the driver (Q53) and the
power (Q54) AMPs to keep the TX output power stable.
• TX AMPLIFIERS AND APC CIRCUIT
D50
Q50
From VCO
LO
BUFF
SW
4 - 2
D52 D90
Q53
Q54
PRE PWR ANT
DRIVE
AM P SW
Q357
IC50
D91
T5V
APC
PWR
CTRL
DET
PCON
ANTENNA
J1
LPF