Transmitter Circuits - Icom IC-F43TR Service Manual

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 of the base band IC (IC14, pin 23)
and are then applied to the low-pass filter section of it.
The filtered signals passes through the high-pass filter to
suppress unwanted harmonic components. The signals
pass through (or bypass) scrambler and expander sections.
The signals are amplified at the amplifier section of the base
band IC (IC14).
The amplified signals pass through the AF volume (R315),
and are then applied to the AF power amplifier (IC15) to
drive the speaker.
4-1-6 RECEIVE MUTE 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 mute switch.
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).
• 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
• CTCSS AND DTCS
The tone squelch circuit detects AF 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) passes 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 inside of the CPU (IC22, pin 46)
via the "CDEC" line. The CPU outputs AF mute control sig-
nal and AF power supply circuits (Q41, Q42) control signals
via the "AFON" 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-
posed of microphone amplifier, compressor, scrambler, lim-
iter, splatter filter, MSK modulator, etc. at the microphone
amplifier section.
The AF signals (MIC) from the microphone (MC1) are
applied to the amplifier (IC23, pins 6, 7). The amplified sig-
nals are applied to the microphone amplifier section of the
base band IC (IC14, pins 3, 4). The amplified signals are
passed through or bypass the compressor, scrambler sec-
tions of IC14, and are then passed through the high-pass,
limiter amplifier, splatter filter sections of IC14.
The filtered AF signals are applied to the FM/PM switch
(IC13, pin 6), and pass through the low-pass filter (IC6, pin
2).
The filtered signals are applied to the D/A converter (IC12,
pin 4). The output signals from the D/A converter (IC12, pin
3) are applied to the modulation circuit (D12).
IC23
volume
LPF
IC23
AMP
D12
to TX VCO circuit
(Q16, D13)
4 - 2
AF
IC15
AF
Speaker
AMP
Microphone