Icom IC-M45 Service Manual Download Page 8

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3 - 2

3-1-6 SQUELCH CIRCUIT

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] control pot, and are then
applied to the active filter section (IC1, pin 8). The active fil-
ter section amplifies and filters noise components. The fil-
tered signals are applied to the noise detector section and
output from pin 13 as the “SQL” signal. The “SQL” signal is
applied to the CPU (LOGIC unit; IC1, pin 22). The CPU ana-
lyzes the noise condition and outputs the RMUT signal via
the I/O expander IC (IC7) to toggle the analog switches (IC5,
pins 4, 3) as an AF mute switch.

3-1-7 WEATHER ALERT DECODER CIRCUIT

[USA version only]

When the weather alert function is activated and a 1050 Hz
alert tone from an NOAA weather radio broadcast is
received, the IC-M45 emits beep tones and indicates flash-
ing “ALT” on the display to inform of an emergency weather
report on the air.

AF signals from the FM IF IC (IC1, pin 9) are applied to the
tone decoder (IC10, pin 3). When a 1050 Hz signal is detect-
ed, the tone decoder outputs a low level signal from pin 8
and the output signals are applied to the CPU (LOGIC unit;
IC1) to control beep tones and the “ALT” indicator.

3-2 TRANSMITTER CIRCUITS

3-2-1 MICROPHONE AMPLIFIER CIRCUIT

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 (IC4). A capacitor (C135) and resistor
(R114) are connected to the amplifier to obtain the pre-
emphasis characteristics.

The amplified signals are applied to the IDC amplifier (IC6a,
pin 2) via the analog switch (IC5, pins 8, 9) and are passed
through the splatter filter (IC6b) to suppress unwanted 3 kHz
or higher signals. The filtered signals are then applied to the
modulation circuit.

3-2-2 MODULATION CIRCUIT

The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.

Audio signals from the splatter filter (IC6b) pass through the
frequency deviation adjustment pot (R130) and are then
applied to the modulation circuit (D7) to change the reac-
tance of D7, and modulate the oscillated signal at the TX-
VCO (Q4).

3-2-3 DRIVE AMPLIFIER CIRCUIT

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 T/R switch (D9). The transmit signal from
the T/R switch is amplified to the pre-drive (Q10) and drive
(Q11) amplifiers to obtain an approximate 400 mW signal
level. The amplified signal is then applied to the RF power
amplifier (IC2).

3-2-4 POWER AMPLIFIER CIRCUIT

The power amplifier circuit amplifies the driver signal to an
output power level.

IC2 is a power module which has amplification output capa-
bilities of about 35 W. The output from IC2 (pin 4) is passed
through the antenna switching circuit (D13) and is then
applied to the antenna connector via the low-pass filter.

3-2-5 APC CIRCUIT

The APC circuit stabilizes transmit output power.

The RF output signal from the power amplifier (IC2) is
detected at the power detector circuit (D11, D12, L22) and is
then applied to one of the differential amplifier inputs (Q13,
pin 5) via the High/Low control circuit (R70, R71, Q16). The
applied voltage controls the differential amplifier output
(Q17, pin 2) and the bias voltage control (Q12). Thus the
APC circuit maintains a constant output power.

The reflected power from the antenna connector is detected
at D12 and is then applied to the CPU. The detected voltage
increases when the antenna is mismatched, causing the
output power to be switched from High to Low to protect the
power module (IC2).

D11

D12

L22

TMUT

HI/LO

"TXDET" signal
to the CPU

Q12

Q13

Q15

Q16

R71

R72