Transmitter Circuits - Icom IC-F50 Service Manual Addendum

4-1-5 AF AMPLIFIER CIRCUIT
(MAIN AND FRONT UNITS)
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 (IC1, pin 9) are amplified
at the AF amplifier section of the base band IC (IC10, pin 5)
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,
and are then applied to (or bypass) the scrambler IC (IC14)
via the analog switch (IC13). The signals are amplified at
the amplifier section of the base band IC (IC10), and pass
through the AF mute switch (IC406) and low-pass filter
(IC403). The filtered signals pass through the AF volume,
and are then applied to the AF power amplifier (IC405) to
drive the speaker.
4-1-6 RECEIVE MUTE CIRCUITS
(MAIN AND FRONT UNITS)
• 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 (IC1, pin 9) are passed through the D/A converter (IC6,
pin 1). The signals are applied to the active filter section in
the FM IF IC (IC1, pin 8). Noise components about 10 kHz
are amplified and output from pin 7.
The filtered signals are converted into the pulse-type
signals at the noise detector section and output from pin 13
(NOIS).
The "NOIS" signal from the FM IF IC is applied to the CPU
(FRONT unit; IC401, pin 41). Then the CPU analyzes the
noise condition and controls the AF mute signal via "AFON"
line from expander IC (FRONT unit; IC410, pin 7) to the AF
power controller (FRONT unit; Q401, Q402).
• AF AMPLIFIER AND MICROPHONE AMPLIFIER CIRCUITS
"DET" AF signal
from FM IF IC (IC1, pin 9)
20
18 18
Base band IC
5
(IC10)
19
• 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 (IC1,
pin 9) passes through the low-pass filter (IC5, pin 5) to
remove AF (voice) signals, and are then applied to the
amplifier (MAIN unit; IC5, pin 10). The amplified signals
are applied to the CTCSS or DTCS decoder inside of the
CPU (FRONT; IC401, pin 44) via the "CDEC" line. The CPU
outputs AF mute control signal, and is then applied to the
I/O expander IC (IC410). The IC outputs AF mute circuit
(IC406) and AF power supply circuits (Q401, Q402) control
signals via the "AFON" line.

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(FRONT AND MAIN UNITS)
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
composed of microphone amplifier, compressor, scrambler,
limiter, splatter filter, MSK modulator, etc. at the microphone
amplifier section.
The AF signals (MIC) from the microphone (MC401) are
passed through the microphone mute switch (IC406, pins
2, 1), and are then applied to the amplifier (IC407, pins
2, 6). The amplified signals pass through (or bypass) the
scrambler IC (IC14) via the analog switch (IC13), and are
then applied to the microphone amplifier section of the
base band IC (MAIN unit; IC10, pins 3, 4). The amplified
signals are passed through or bypass the compressor,
scrambler sections of IC10 (MAIN unit), and are then
passed through the high-pass, limiter amplifier, splatter
filter sections of IC10 (MAIN unit).
The filtered AF signals are applied to the FM/PM switch
(MAIN unit; IC11, pin 6), and pass through the low-pass
filter (MAIN unit; IC5, pin 1). The amplified signals are
applied to the D/A converter (MAIN unit; IC6, pin 4)
The output signals from the D/A converter (MAIN unit; IC6,
pin 3) are applied to the modulation circuit (MAIN unit;
D18).
AF
IC403
volume
AF mute
LPF
(IC406)
Scrambler IC
15 8
(IC14)
11
Analog switch
4 - 2
IC405
Speaker
AF
(SP1)
10
9
(IC13)