Icom IC-M21 Service Manual - page 10
4 - 2
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 90). 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 10 and 8). The filtered audio is applied to the
modulation circuit 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 D22 and
D23 to modulate an oscillated signal at the VCO circuit
(Q21, Q22, D22, D23). The oscillated signal is amplified at
the buffer-amplifiers (Q23, Q24).
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 (L80–L82, C80–C86) 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 (Automatic Power Control) circuit provides stable
output power from the power amplifier even when the input
voltage or temperature changes, and selects HIGH, MID-
DLE or LOW (1W type selects HIGH or LOW) output power.
The APC circuit consists of a power detector and APC con-
trol 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 APC
control circuit.
• APC CONTROL CIRCUIT (MAIN UNIT)
The “TDETV” signal from the power detector circuit is
applied to the CPU (IC360, pin 92) to control the input volt-
age of the pre-driver (Q53) and the power amplifier (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 con-
troller (IC50) to provide stable output power.
I. SENS
Q53
IC50
Q51, Q52
APC ctrl
To the 1st IF circuit
PWR amp.
Pre-Drive
D80, C87
Q54
D52
D91
Ant SW
Ant
D90
+
"PCON" signal
from IC190 (pin 3)
"TDETV" signal
to CPU (pin 92)
The amplified signal from the
buffer amplifier (Q50)
"VCC"
"T5V"
PWR det.
TX
RX
• APC CIRCUIT