Transmitter Circuits - Icom IC-F43GT Service Manual

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The 2nd IF signal from the 2nd mixer (IC9, pin 3) passes
through the ceramic filter (FI2) to remove unwanted hetero-
dyned frequencies. It is then amplified at the limiter amplifi-
er section (IC9, pin 5) and applied to the quadrature detec-
tor section (IC9, pins 10, 11) to demodulate the 2nd IF sig-
nal into AF signals.
The demodulated AF signals are output from pin 9 (IC9) and
applied to the base band IC (IC14).
4-1-5 AF AMPLIFIER CIRCUIT (MAIN UNIT)
The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker. This transceiver employs the base
band IC which is composed of pre-amplifier, expander,
scrambler, MSK de-modulator, etc. at the AF amplifier sec-
tion.
The AF signals from the FM IF IC (IC9, pin 9) are amplified
at the AF amplifier section in the base band IC (IC14, pin
23), and are then applied to the high-pass filier and low-pass
filter section of it.
The filtered signals pass through the high-pass filter to sup-
press unwanted harmonic components. The signals pass
through (or bypass) scrambler and expander sections. The
signals are amplified at the amplifier section in the base
band IC (IC14).
The output siganls from IC14 (pin 20) pass through the low-
pass filter sector (IC23), and are then applied to the AF vol-
ume (R315), and AF power amplifier (IC15).
4-1-6 SQUELCH CIRCUITS (MAIN UNIT)
• NOISE SQUELCH
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 amplifier contrler.
• AF AND MIC AMPLIFIER CIRCUIT
"DET" AF signal
from FM IF IC (IC9, pin 9) 23
IC13
FM/PM switch
IC6
LPF
"CTCSS/DTCS" signal from
D/A conveter IC (IC12, pin 11)
"TONE" signal from CPU via low-pass
filter (IC22, pin 43)
20
Base band IC
(IC14)
7
3
IC12
4
3
FM mod.
D/A converter
Some noise components in the AF signals from the FM IF IC
(IC9, pin 9) are passed through the D/A converter (IC12, pin
1). The signals are applied to the active filter section in the
FM IF IC (IC9, pin 8). The active filter section filters and
amplifies noise components. The amplified signals are con-
verted into the pulse-type signals at the noise detector sec-
tion. The detected signals output from pin 13 (NOIS) via the
noise comparator section.
The "NOIS" signal from the FM IF IC is applied to the CPU
(IC22, pin 75). Then the CPU analyzes the noise condition
and outputs the AF mute signal as "AFON" from the pin 70
to the AF power controller (Q41, Q42).
• CTCSS AND DTCS
The tone squelch circuit detects tone signals and opens the
squelch only when receiving a signal containing a matching
subaudible tone (CTCSS or DTCS). When tone squelch is in
use, and a signal with a mismatched or no subaudible tone
is received, the tone squelch circuit mutes the AF signals
even when noise squelch is open.
A portion of the "DET" AF signals from the FM IF IC (IC9, pin
9) pass through the low-pass filter (IC19, pin 5) to remove
AF (voice) signals, and are then applied to the amplifier
(IC19, pin 3). The amplified signals are applied to the
CTCSS or DTCS decoder in the CPU (IC22, pin 46) via the
"CDEC" line. The CPU outputs AF mute control signal and
tone mute circuit (Q502) controls via the "MUTE" line.

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN UNIT)
The microphone amplifier circuit amplifies audio signals
within +6 dB/octave pre-emphasis characteristics from the
microphone to a level needed for the modulation circuit.
This transceiver employs the base band IC which is com-
IC23
volume
LPF
IC23
AMP
D12
to TX VCO circuit
(Q16, D13)
4 - 2
AF
IC15
AF
Speaker
AMP
Microphone

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