Icom IC-F310 Service Manual Download

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4 - 2

4-1-5 AF AMPLIFIER CIRCUIT (MAIN unit)

The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker.

AF signals from the FM IF IC (IC3, pin 9) are amplified at the
AF amplifier (IC7a) and then pass through the AF switching
IC (IC4, pins 1, 2) and high-pass filter (IC8) whose charac-
teristics are controlled by the “AFHPF” line. When “AFHPF”
is at a high level, the cut off frequency is shifted higher to
remove CTCSS or DTCS signals.

The filtered signals from IC8 (pin 7) are amplified at the lim-
iter (IC7b) and buffer (IC7c) amplifiers, and passed through
the de-emphasis circuit (R145, C182) with frequency char-
acteristics of –6 dB/octave, and are then applied to the level
controller (IC5). The audio level controlled signals are
passed through the low-pass filter (IC6b) and AF switching
IC (IC4, pins 8, 9), and are then power amplified at the AF
amplifier (IC10) to drive a speaker via the buffer amplifier
(IC6d).

4-1-6 RECEIVER MUTE CIRCUITS

(MAIN and FRONT units)

•

NOISE SQUELCH

The noise 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.

A portion of the AF signals from the FM IF IC (IC3, pin 9) are
applied to the level controller (IC5, pin 24). The level con-
trolled noise components are output from pin 23 and are
applied to the active filter in IC3 (pin 8). Noise components
of about 10 kHz are amplified and output from pin 7 and are
then applied to the noise detector section (pins 10, 11). The
detected noise signals are rectified and output from pin 13
without smoothing.

The noise signal (NOIS) from IC3 (pin 13) is applied to the
CPU (FRONT unit; IC1, pin 19). The CPU analyzes the
noise condition and outputs the RMUT signal via the I/O
expander IC (IC11) to toggle the AF mute switches (IC4a/c).

• 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 AF signals from the AF amplifier (IC7a)
passes through the low-pass filter (FRONT unit; Q5) to
remove AF (voice) signals and is applied to the CTCSS or
DTCS decoder inside the CPU (FRONT unit; IC1, pin 97) via
the “CTCIN” line to control the AF mute switch via the I/O
expander IC (IC11).

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT

(MAIN unit)

The microphone amplifier circuit amplifies the audio signals
from the microphone, 6 dB/octave pre-emphasis
characteristics, to a level needed for the modulation circuit.

The AF signals from the microphone are amplified at the AF
amplifier (IC7d) and are then passed through the pre-
emphasis circuit (R172, C295) which has +6 dB/octave pre-
emphasis characteristics.

The pre-emphasized signals are applied to the AF switching
IC (IC4, pins 4, 3), and are then passed through the high-
pass filter (IC8a/b). The filtered signals are amplified at the
limiter (IC7b) and buffer (IC7c) amplifiers.

The signals are applied to the level controller (IC5, pins 16,
15). The deviation level controlled signals are passed
through the splatter filter (IC6b) and AF switching IC (IC4,
pins 11, 10), and are then applied to modulation circuit as
the “MOD” signal.

The narrow/wide switch (Q21) is connected to the input of
the splatter filter (IC6b) and switched by the “NWC” signal
coming from the I/O expander IC (IC11). When “NWC” is at
a high level, the narrow/wide switch (Q21) shifts the filter
cut-off frequency for narrow deviation selection.

4-2-2 MODULATION CIRCUIT (MAIN unit)

The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone AF signals.

The “MOD” signals from the AF switching IC (IC4, pin 10)
change the reactance of D9 to modulate the oscillated sig-
nal at the VCO circuit (Q7, Q8). The modulated signal is
amplified at the buffer amplifiers (Q6, Q4) and is then
applied to the drive amplifier circuit.

• AF circuit

Speaker

RMUT

HPF

Volume control signal

Amp.

AFHPF

AF mute

switch

AF mute

switch

AF power