Transmitter Circuits - Icom IC-F24/S Service Manual Addendum

4-1-5 AF AMPLIFIER CIRCUIT
The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker.
The AF signals from the FM IF IC (IC1, pin 9) pass through
the high-pass filter (IC6, pins 3 and 1) to suppress unwant-
ed harmonic components. The signals pass through the
RX mute switch (Q34) which is controlled by "RMUT" sig-
nal from the CPU (IC13, pin 56), and are then applied to
another high-pass filter (IC6, pins 13 and 14). The filtered
signals pass through the low-pass filter (IC6, pins 6 and
7) via the analog switch (IC10, pins 1 and 2). The signals
are applied to the analog switch (IC10, pin 10) again, and
are then applied to the AF power amplifier (IC12, pin 4) via
the AF volume (R226). The amplified AF signals are output
from pin 10, and are then applied to the internal speaker
which is connected to J1 via the [SP] jack (J3).
4-1-6 RECEIVE MUTE CIRCUITS
• NOISE SQUELCH
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.
Some noise components in the AF signals from the FM IF
IC (IC1, pin 9) are applied to the D/A converter (IC8, pin
1) as "DET" signal, and are then output from pin 2. The
signals are applied to the active filter section in the FM IF
IC (IC1, 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 and
output from pin 13 as "NOIS" signal.
The "NOIS" signal from the FM IF IC is applied to the CPU
(IC13, pin 53). Then the CPU analyzes the noise condition
and outputs the AF mute control signal from the CPU (pin
56) as "RMUT" signal from pin 56. The signal is applied to
the RX mute switch (Q34) to control the AF signal muting.
• ANALOG SWITCHING CIRCUITS
"DET" AF signal
from FM IF IC (IC1, pin 9)
IC6 B
LPF
"CTCSS/DTCS" signal from
D/A conveter IC (IC8, pin 10)
"TONE" signal from CPU (IC13) via low-pass
filters (IC5 A / B pin 7)
IC6 A
HPF
MUTE
1
IC6 C
Analog SW
4
9
(IC10)
10
11
8
D18
IC8
FM mod.
D/A converter
• CTCSS AND DTCS
The tone squelch circuit detects tone signals and opens the
squelch only when the receiving signal contains matched
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 sig-
nals even when noise squelch is open.
A portion of the "DET" signals from the FM IF IC (IC1, pin
9) passes through the low-pass filter (IC7, pins 5 and 7)
to remove AF (voice) signals, and are then applied to the
amplifier (IC7, pin 3). The amplified signals are applied to
the CTCSS or DTCS decoder inside of the CPU (IC13, pin
60) as the "CDEC" signal. The CPU outputs AF mute con-
trol signal from pin 56, and is then applied to the RX mute
switch (Q34) and analog switch (IC10, pins 12 and 13) to
control AF signals muting as "RMUT" signal.

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT
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.
The AF signals from the microphone are passed through
the microphone mute switch (Q35), and are then applied
to the amplifier (IC6, pins 9 and 8) via the high-pass filter
(IC6, pins 13 and 14). The amplified signals are applied to
the analog switch (IC10, pin 4), and outputs from pin 3. The
signals pass through the low-pass filter (IC6, pins 6 and 7),
then applied to the analog switch (IC10, pin 9) again and
output from pin 8.
The signals are applied to the D/A converter (IC8, pin 4).
The converted signals output from pin 3, and applied to the
modulation circuit (D18) as "MOD" signal.
Q34
RX
IC6 D
HPF
AMP
AF
volume
IC12
AF
AMP
R226
to TX VCO circuit
(Q13, D17, D21)
4 - 2
Q35
MIC
Microphone
MUTE
Speaker