Transmitter Circuits - Icom IC-M604 Service Manual

4-1-5 AF AMPLIFIER CIRCUIT (AF UNIT)
The AF amplifier circuit amplifies the demodulated signals
to drive a speaker. The AF circuit includes an AF mute cir-
cuit for the squelch.
AF signals from the FM IF ICs (channel 70; IC6, pin 9, other
channels IC1, pin 9) are passed through the analog switch
(IC7, pins 10, 11) via the "DET" signal, and are applied to
the de-emphasis circuit (R31, C41). The de-emphasis circuit
is an integrated circuit with frequency characteristic of –6
dB/octave.
The signals pass through the bandpass filter (Q11, Q12),
and are then applied to the AF mute swtich (Q13). The sig-
nals passed through the [VOLUME] control (VR unit; R1),
and are then applied to the AF power amplifier (IC3, pin
1) to obtain 5 W AF audio output power. The amplified AF
signals drive the internal speaker as "SP+" signal directly or
external speaker as "AF" signal via the RL2.
4-1-6 SQUELCH CIRCUIT
(MAIN AND LOGIC UNITS)
A squelch circuit cuts out AF signals when no RF signals
are received. By detecting noise components in the AF sig-
nals, the squelch circuit switches the AF mute switch.
A portion of the AF signals from the FM IF IC (IC1, pin 9)
is passed through C89, and is applied to the D/A converter
(IC15, pin 24) to control the amplitude. The signal is applied
to the FM IF IC's active filter section (IC1, pin 8). The active
filter section amplifies and filters noise components. The fil-
tered signals are applied to the noise detector section and
output from pin 14 as the "SQL" signal. The "SQL" signal is
amplified at the DC amplifier (IC2) and applied to the main
CPU (LOGIC unit; IC1, pin 104) as the "SQL" signal. The
main CPU compares "SQL" voltage with "SQLV" voltage from
the SQL board, and outputs the "MICM" and "RMUTE" sig-
nals to toggle the AF mute switches (Q7, Q13).

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(AF 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.
• USING HM-126RB/RG
The AF signals from the microphone (ACC unit; HM-126RB/
RG) are amplified at the microphone amplifier (IC6a) via the
analog switch (IC4, pins 11, 10) as "MIC" signal. A capacitor
(C77) and resistor (R73) are connected to the microphone
amplifier to obtain the pre-emphasis characteristics.
• USING HM-162
The AF signals from the microphone (ACC unit; HM-162)
are amplified at the microphone amplifier (IC6a) via the
analog switch (connecting option1 jack: IC16, pins 2, 3, 4;
connecting option2 jack: IC16, pins 4, 9, 10) as "AF/MIC1"
or "AF/MIC2" signals. A capacitor (C77) and resistor (R73)
are connected to the microphone amplifier to obtain the pre-
emphasis characteristics.
The amplified signals are applied to the IDC amplifier (IC8a,
pin 2) via the analog switch (IC7, pins 2, 3, 9), and are then
passed through the splatter filter (IC8b) to suppress unwant-
ed 3 kHz or higher signals. The filtered signals are 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.
The audio signals from the splatter filter (IC8b) are passed
through the D/A converter IC (IC15, pins11, 12), and are
then applied to the modulation circuit. The applied signals
change the reactance of the varactor diode (D37), and mod-
ulate the oscillated signal at the TX-VCO (Q18).
4-2-3 PRE-DRIVE AND YGR AMPLIFIERS CIRCUIT
(MAIN UNIT)
The drive amplifier circuit amplifies the VCO oscillating sig-
nal to a level needed at the power amplifier.
The output signal from VCO circuit is amplified at the buffer
amplifiers (Q19 and Q27), and is applied to the TX/RX
switch (D43). The transmit signal from the TX/RX switch is
amplified at the pre-drive (Q28) and YGR (Q30) amplifiers
to obtain an approximate 50 mW signal level. The amplified
signal is then applied to the RF power amplifier (IC13).
4-2-4 POWER AMPLIFIER CIRCUIT (MAIN UNIT)
The power amplifier circuit amplifies the driver signal to an
output power level.
IC13 is a power module which has amplification output
capabilities of about 35 W with 50 mW input. The output sig-
nal from IC13 (pin 1) is passed through the antenna switch-
ing circuit (D46) and is then applied to the antenna connec-
tor via the low-pass filter (L1, L2, L89, C361, C364).
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