Transmitter Circuits - Icom IC-M32 Service Manual

4-1-5 SQUELCH CIRCUIT (MAIN UNIT)
The noise 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 divided by C175 from the FM IF
IC (IC170, pin 9) are applied to the D/A convertor (IC190, pin
16) to adjust amplitude. The signals from the D/A convertor
(IC190, pin 15) are applied to the active filter section (IC170,
pin 8). The active filter section amplifies and filters noise
components. The filtered signals are applied to the noise
detector section and output from IC170 (pin 14) as the
"NOISV" signal.
The "NOISV" signal from IC170 (pin 14) is applied to the
CPU (IC360, pin 2). The CPU compares the set squelch
level voltage and "NOISV" signal voltage to control the
squelch output.

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 characteristics from the micro-
phone to a level needed for the modulation circuit.
The AF signals from the microphone are passed through the
pre-emphasis circuit (R253, C254) and are then applied to
the microphone amplifier (IC200b). The amplified AF signals
are applied to analog switch (IC260, pin 5).
The AF signals are amplified again at the limiter-amplifier
(IC200a) and then passed through the low-pass filter
(IC200d, pins 12 and 13). The filtered audio is applied to the
RF unit as the "MOD" signal.
4-2-2 MODULATION CIRCUIT (MAIN UNIT)
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
The audio signals "MOD" change the reactance of D20 to
modulate an oscillated signal at the VCO circuit (Q21, Q22,
D22, D23). The oscillated signal is amplified at the buffer-
amplifiers (Q23, Q24).
APC CIRCUIT
"TDETV" signal to the CPU
IC360 (pin 4)
"TXMS" signal to the CPU
IC360 (pin 11)
"T5V"
The amplified signal from the
buffer amplifier (Q50)
APC ctrl
"PCON" signal
+
from IC190 (pin 3)
IC50
TX MUTE SW
Q351
To the 1st IF circuit
4-2-3 DRIVE/POWER AMPLIFIER CIRCUITS
(MAIN UNIT)
The signal from the VCO circuit passes through the trans-
mit/receive switching circuit (D50, D51) and is applied to the
buffer-amplifier (Q50). The amplified signal is amplified by
the pre-driver (Q53) and the power amplifier (Q54) to obtain
5 W of RF power (at 7.2 V). The amplified signal passes
through the antenna switching circuit (D52), and low-pass fil-
ter (L81, L82, C83–C86, C89) and is then applied to the
antenna connector.
The bias current of the pre-driver (Q53) and the power
amplifier (Q54) is controlled by the APC circuit to stabilize
the output power.
4-2-4 APC CIRCUIT (MAIN UNIT)
The APC circuit provides stable output power from the
power amplifier even when the input voltage or temperature
changes, and, selects HIGH or LOW output power. The APC
circuit consists of a power detector and APC control circuits.
• POWER DETECTOR CIRCUIT (MAIN UNIT)
The power detector circuit (D80, C87) detects the transmit
output power level and converts it to DC voltage as the
"TDETV" signal. The detected signal is applied to the TX
control circuit.
• APC CONTROL CIRCUIT (MAIN UNIT)
The detected signal from the power detector circuit (D91,
R280) is applied to the CPU (IC360, pin 4) to control the
input voltage of the pre-driver (Q53) and the power amplifi-
er (Q54). When the output power changes, the CPU (IC360)
outputs APC control signal to the D/A converter (IC190). And
then "PCON" signal from the D/A converter controls the APC
controller (IC50) to provide stable output power.
PWR det.
D80, C87
Pre-Drive PWR amp.
Q53 Q54
D91
4 - 2
Ant
TX
D52
RX
D90
Ant SW