Transmitter Circuits - Icom IC-M401EURO Service Manual

4-1-5 AF AMPLIFIER CIRCUIT
(MAIN AND FRONT UNITS)
The AF amplifier circuit amplifies the detected signals to
drive a speaker. The AF circuit includes an AF mute circuit
for the squelch.
AF signals from IC1 (pin 9) are applied to the de-emphasis
circuit (R101, C140). The de-emphasis circuit is an integrat-
ed circuit with frequency characteristic of –6 dB/octave.
The integrated signals pass through the high-pass filter
(Q21, R96–R100, C138, C139) to suppress unwanted lower
noise signals.
The filtered signals are applied to the AF mute circuit
(IC12A), and then passes through the [VOLUME] control
(VR-A board; R1) via the "VOL1" signal. The signals are
applied to the AF power amplifier (IC9, pin 1). The output
signal from IC9 (pin 4) drives the internal (external) speaker.
4-1-6 SQUELCH CIRCUIT
(MAIN UNIT AND LOGIC BOARD)
A squelch circuit cuts out AF signals when no RF signals are
received. By detecting noise components in the AF signals,
the squelch circuit switches the AF mute switch.
A portion of the AF signals from the FM IF IC (IC1, pin 9)
pass through the squelch adjustment pot (R16), and are
then applied to the active filter section (IC1, pin 8). The
active filter section amplifies and filters noise components.
The filtered signals are applied to the noise detector section
and output from pin 14 as the "SQL" signal. The "SQL" sig-
nal is applied to the CPU (LOGIC board; IC1, pin 90) after
being passed through the amplifier (IC13, pins 1, 4). The
CPU analyzes the noise condition and outputs the
"RMUTM", "PTTM" signals to toggle the AF mute switches
(IC12A, IC12B).

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN UNIT)
The microphone amplifier circuit amplifies audio signals with
+6 dB/octave pre-emphasis from the microphone to a level
needed at the modulation circuit.
The AF signals from the microphone are amplified at the
microphone amplifier (IC11A) via the analog switch (IC12B,
pins 6, 5). A capacitor (C159) and resistor (R121) are con-
nected to the amplifier to obtain the pre-emphasis charac-
teristics.
• AF AMPLIFIER CIRCUIT
External SP
Internal SP
(LOGIC board)
Vcc
(VR-A board)
PWR
AMP
AF
MUTE
IC9
4 - 2
The amplified signals are passed through the MIX circuit
(IC10A, pins 2, 1) to do amplitude limiting. and are then
applied to the splatter filter (IC10B, pin 5) to suppress
unwanted 3 kHz or higher signals. The filtered signals are
then applied to the modulation circuit.
4-2-2 MODULATION CIRCUIT (MAIN UNIT)
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
Audio signals from the splatter filter (IC10B) pass through
the frequency deviation adjustment pot (R138) and are then
applied to the modulation circuit (D2, D3, D4) to change the
reactance of D2, D3, D4 and modulate the oscillated signal
at the VCO (Q5, Q6).
4-2-3 DRIVE AMPLIFIER CIRCUIT (MAIN UNIT)
The drive amplifier circuit amplifies the VCO oscillating sig-
nal to a level needed at the power amplifier.
The VCO output is buffer-amplified by Q7 and Q9, and is
then applied to the Tx/Rx switch (D6). The transmit signal
from the Tx/Rx switch is amplified to the pre-drive (Q10) and
YGR (Q11) amplifiers to obtain an approximate 300 mW sig-
nal level. The amplified signal is then applied to the RF
power amplifier (IC3).
4-2-4 POWER AMPLIFIER CIRCUIT (MAIN UNIT)
The power amplifier circuit amplifies the driver signal to an
output power level.
IC3 is a power module which has amplification output capa-
bilities of about 35 W with 300 mW input. The output from
IC3 (pin 4) is passed through the antenna switching circuit
(D10) and is then applied to the antenna connector via the
low-pass filter.
4-2-5 APC CIRCUIT (MAIN UNIT)
The APC circuit stabilizes transmit output power.
The RF output signal from the power amplifier (IC3) is
detected at the power detector circuit (D8, D9, L12) and is
then applied to one of the differential amplifier inputs (Q15,
pin 5) via the High/Low control circuit (R71, Q16). The
applied voltage controls the differential amplifier output
(Q15, pin 2) and the bias voltage control (Q12). Thus the
APC circuit maintains a constant output power.
de-emphasis
circuit
R101
HPF
"DET" signal from the
C140
FM IF IC (IC1, pin 9)