Transmitter Circuits - Icom IC-M88 Service Manual

4-1-5 SQUELCH CIRCUIT
(MAIN AND LOGIC UNITS)
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 C242 from the
FM IF IC (IC231, pin 9) are applied to the D/A converter
(IC251, pin 2) to adjust amplitude. The signals from the
D/A converter (IC251, pin 1) are applied to the active filter
section (IC231, pin 8, R239–R241, C237, C238). The active
filter section amplifies and filters noise components. The
filtered signals are applied to the noise detector section
and output from IC231 (pin 14) as the "NOISV" signal.
The "NOISV" signal from IC231 (pin 14) is applied to the
CPU (LOGIC unit; IC611, pin 32). 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
(LOGIC AND MAIN UNITS)
The microphone amplifier circuit amplifies audio signals
with +6 dB/octave pre-emphasis characteristics from the
microphone to a level needed for the modulation circuit.
• In case of the internal microphone
The AF signals from the internal microphone (MC461) is
applied to the microphone amplifier (IC471a, pin 6) via the
"INMIC" signal.
• In case of external microphone
The AF signals from the external microphone (CP458) is
applied to the microphone amplifier (IC471a, pin 6) via the
"EXTMIC" signal.
The amplified signals are passed through the pre-emphasis
circuit (R463, C463) and are then applied to the AF mute
switch (IC481a, pin 1) via the "MICO" signal after being
passed through the another microphone amplifier (IC471b,
pins 2 and 1).
The AF signals are amplified again at the limiter-amplifier
(IC491a, pin 2) and then passed through the low-pass filter
(IC491b, pins 6 and 7). The filtered audio passes through
the analog swtich (MAIN unit; IC251, pins 4 and 3), and is
then applied to the MAIN unit as the "MOCON" signal.
• APC CIRCUIT
VCC
T5V
S5V
"TXMS" signal from the
expander IC (IC341, pin 12)
"T1CON" signal from the D/A
convertor IC (IC251, pin 14)
RF signal
Q91
from PLL
Buffer
amp.
3
Q141 IC141
1
"TDETV" to the CPU
APC control circuit
(LOGIC unit; IC661, pin 31)
4 - 2
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 "MOCON" change the reactance of D39
to modulate an oscillated signal at the TX VCO circuit (Q51,
D35–D38, L34, L51, C32, C33, C52–C54). The oscillated
signal is amplified at the buffer-amplifiers (Q61, Q62).
4-2-3 PRE-DRIVE/POWER AMPLIFIER CIRCUITS
(MAIN UNIT)
The signal from the VCO circuit passes through the
transmit/receive switching circuit (D91, D92) and is applied
to the buffer-amplifier (Q91). The amplified signal is
amplified by the pre-driver (Q101) and the power amplifier
(Q101) to obtain 5 W of RF power (at 7.2 V). The amplified
signal passes through the antenna switching circuit (D131),
and low-pass filter (L131, L132, C131–C136) and is then
applied to the antenna connector.
The bias current of the buffer amplifier (Q91), pre-driver
(Q101) and power amplifier (Q111) 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, MIDDLE, LOW or EXTRA
LOW output power. The APC circuit consists of a power
detector and APC control circuits.
• POWER DETECTOR CIRCUIT (MAIN UNIT)
The power detector circuit (D132) detects the transmit
output power level and converts it to DC voltage as the
"TDETV" signal. The detected signal is applied to the APC
control circuit.
• APC CONTROL CIRCUIT (MAIN AND LOGIC UNITS)
The "TDETV" signal from the power detector circuit is
applied to the CPU (LOGIC unit; IC661, pin 31) to control
the input voltage of the buffer amplifier (Q91), pre-driver
(Q101) and power amplifier (Q111). When the output power
changes, the CPU (LOGIC unit; IC661) outputs APC control
signal to the D/A converter (IC190). And then "T1CON"
signal from the D/A converter controls the APC controller
(IC141) to provide stable output power.
Q101 Q111
Power
Pre-drive
amp.
amp.
antenna
D131
ANT
LPF
SW
Power detector
circuit (D132)