Transmitter Circuits - Icom IC-M23 Service Manual

4-2 TRANSMITTER CIRCUITS

TX AF CIRCUITS
AF signal from the microphone (MIC signals) is passed
through the MIC mute SW (IC260A), amplifi ed by MIC AMP
(IC200B), and then applied to the D/A converter (IC190),
through another MIC mute SW (IC260C).
The level-adjusted MIC signal is pre-emphasized by R201
and C385 to obtain +6 dB/oct of frequency response, and
then passed through the limiter (IC200A) to prevent over
deviation. The amplitude-limited MIC signal is fi ltered by the
splatter fi lter (IC200D) to remove 3 kHz and higher signals,
and then applied to the VCO (Q21, Q22, D22) for frequency
modulation.
• TX AF AND MODULATION CIRCUITS
Q 21,Q22
D22
MOD
MUT E
MOD
IC200D
LPF
MC250
MIC
IC260A
INMIC MIC
MUTE
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 (Q21, Q22,
D22) is passed through the buffers (Q23 and Q24) and LO
SW (D50), and then sequentially amplified by the buffer
AMP (Q50) and pre-drive AMP (Q53).
The amplifi ed signal is splitted by the power splitter (L100,
L101, R463, C445, C446) and applied to the power AMPs
(Q54 and Q55). The amplifi ed signals are combined at the
power combiner (L107, L108, R465, C457, C458) to obtain
the TX output power.
The amplified TX signal is passed through the ANT SW
(D52) and LPF, and then applied to the antenna.
APC CIRCUIT
A portion of TX output signal is rectifi ed by D91 to produce
the DC voltage which corresponds to the TX power level.
The APC AMP (IC50) compares the voltage and TX power
setting voltage from the D/A converter (IC190). The resulting
voltage from the APC AMP controls the gain of both driver
AMP (Q53) and power AMPs (Q54 and Q55), to keep the TX
output power stable.
• TX AMPLIFIERS AND APC CIRCUIT
D50
Q50
From VCO
LO PRE
BUFF
DRIVE
SW
4 - 2
Q54
PWR
AMP
D52 D90
Q53
PWR PWR
ANT
SPLITTER COMBINER
SW
Q55
PWR
AMP
IC50
Q357
D91
APC PWR
CTRL
DET
PCON
Receive circuits
ANTENNA
J1
LPF