Transmitter Circuits - Icom IC-M25 Service Manual

5-2 TRANSMITTER CIRCUITS

TX AF CIRCUITS
The AF signal from the microphone (MIC signals) is passed
through the MIC mute SW (IC260A), amplified 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/octave of frequency response,
and then passed through the limiter (IC200A) to prevent
over deviation. The amplitude-limited MIC signal is filtered by
the splatter filter (IC200D), which removes 3 kHz and higher
signals, and then applied to the VCO (Q21, Q22, D22 and
D23) for frequency modulation.
• TX AF AND MODULATION CIRCUITS
MC250
MIC
INMIC
Q23
Q24
To LO switch
BUFF BUFF
VCO
IC190
D/A
PRE
LIMIT
AM P
ENPHA
IC260C
MIC
MUT E
Q 21,Q22
Q23 Q24
D22,D23
BUFF BUFF
Q20
MOD
D 20
MUT E
D21
VCO
MOD
IC190
D/A
IC200D
IC200A
PRE
LIMIT
LPF
AM P
ENPHA
IC260A
IC200B
IC260C
MIC
MIC MIC
MUTE
AMP MUT E
TX AMPLIFIERS
The frequency-modulated signal from the VCO (Q21, Q22,
D22 and D23) 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 amplified signal is splitted by the power splitter (L100,
L101, R463, C445 and C446) and applied to the power
AMPs (Q54 and Q55). The amplified signals are combined
at the power combiner (L107, L108, R465, C457 and C458)
to obtain the TX output power.
The amplified TX signal is passed through the ANT SW
(D52) and LPF (L81, L82, C83–C86 and C89), and then
applied to the antenna.
APC CIRCUIT
To LO switch
A portion of TX output signal is rectified by D91 to produce
the DC voltage which represents the TX power level.
The APC AMP (IC50) compares the voltage and TX power
setting voltage from the D/A converter (IC190). The resulted
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 constant.
• TX AMPLIFIERS AND APC CIRCUIT
D50
From the VCO
LO
SW
5-2
From the VCO
Q54
PWR
AMP
Q50
Q53
PWR PWR
PRE
BUFF
DRIVE
SPLITTER COMBINER
Q55
PWR
AMP
IC50
Q357 D91
APC
PWR
CTRL
DET
PCON
D50
Q50 Q53
LO PRE
BUFF
DRIVE
SW
PCON
ANTENNA
D52,D90
ANT
LPF
SW
Receive circuits
PW
SPLI