Transmitter Circuits - Icom IC-F43GT Service Manual

The 2nd IF signal from the 2nd mixer (IC9, pin 3) passes
through the ceramic filter (FI2) to remove unwanted
heterodyned frequencies. It is then amplified at the limiter
amplifi er section (IC9, pin 5) and applied to the quadrature
detector section (IC9, pins 10, 11) to demodulate the 2nd IF
signal into AF signals.
The demodulated AF signals are output from pin 9 (IC9) and
applied to the base band IC (IC14).
5-1-5 AF AMPLIFIER CIRCUIT (MAIN UNIT)
The AF amplifier circuit amplifies the demodulated AF
signals 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
section.
The AF signals from the FM IF IC (IC9, pin 9) are amplifi ed
at the AF amplifier section in the base band IC (IC14, pin
23), and are then applied to the high-pass fi lier and low-pass
fi lter section of it.
The filtered signals pass through the high-pass filter to
suppress unwanted harmonic components. The signals pass
through (or bypass) scrambler and expander sections. The
signals are amplifi ed at the amplifi er section in the base
band IC (IC14).
The output siganls from IC14 (pin 20) pass through the
lowpass fi lter sector (IC23), and are then applied to the AF
volume (R315), and AF power amplifi er (IC15).
5-1-6 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 amplifi er contrler.
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 fi lter section in the
FM IF IC (IC9, pin 8). The active filter section filters and
amplifies noise components. The amplified signals are
converted into the pulse-type signals at the noise detector
section. The detected signals output from pin 13 (NOIS) via
the noise comparator section.
• AF AND MIC AMPLIFIER CIRCUITS
"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
D12
IC12
4 3
D/A converter FM mod.
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 fi lter (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.

5-2 TRANSMITTER CIRCUITS

5-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN UNIT)
The microphone amplifi er circuit amplifi es 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-
AF
IC23
volume
LPF
IC23
AMP
to TX VCO circuit
(Q16, D13)
5 - 2
IC15
AF
Speaker
AMP
Microphone