Icom IC-F4011 Service Manual Download

Page: 8 / 32

4 - 2

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 with J1 via the [SP] jack.

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 (IC13)
as “RMUT” signal from pin 56. The signal is applied to the
RX mute switch (Q34) to control the AF signal muting.

• CTCSS AND DTCS

The tone squelch circuit detects tone signals and opens the
squelch only when receiving a signal containing a matched
subaudible tone (CTCSS or DTCS). When the 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
control 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
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.

Analog SW

(IC10)

"DET" AF signal
from FM IF IC (IC1, pin 9)

volume

AF
AMP

MIC

MUTE

IC12

Q35

Speaker

R226

IC8