Icom IC-F1610 Service Manual Download

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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) are amplified
at the AF amplifier (IC2) and are then applied to the high-
path filter circuit (IC3a/b). The high-pass filter characteristics
are controlled by the HFSW signal from the CPU (IC20, pin
60). When HFSW signal is high, the cut-off frequency is
shifted higher to remove CTCSS or DTCS signals.

The filtered AF signals from the high-pass filter (IC3, pin 1)
are passed through the de-emphasis circuit (R68, C74) with
frequency characteristics of –6 dB/octave, and are then
applied to the electronic volume controller (IC7, pin 2) via
the AF mute switch (Q6).

The output AF signals from the electronic volume controller
(IC7, pin 9) are applied to the AF power amplifier (IC8) to
drive the speaker.

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 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 active filter section in the
IC (pins 10, 11). The N/W switch (Q62) adjusts the input
noise level to the IC between wide and narrow bandwidths.
When NWC signal which is applied to Q62 is High, band-
width setting is wide.

The noise signals from FM IF IC (IC1, pin 11) are passed
through the level controller (IC5, pins 21, 22) and are then
converted into the pulse-type signals (NOIS) at the noise
detector circuit (Q9, Q10).

The NOIS signal from the noise detector (Q10) is applied to
the CPU (IC20, pin 32). The CPU then analyzes the noise
condition and controls the AMUT (pin 56) and SP (pin 79)
ports to toggle the AF mute switches (Q6, Q7).

• 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 (IC2) pass
through the low-pass filter (Q503) and are then applied to
the CTCSS decoder inside the CPU (IC20, pin 43) via the
CDEC line to control the AMUT and SP ports.

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT

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

AF signals (MIC) from the FRONT unit via J5 (pin 10) are
applied to the audio switch (IC25). While transmitting, the
MCON signal from the CPU is high and the AF signals is
passed through IC25 to the microphone amplifier circuit.

The AF signals from IC25 are applied to the microphone
amplifier (IC22) via the pre-amplifier (IC21b). The amplified
signals are applied to the limiter amplifier in IC22.

The entered signals are pre-emphasized with +6dB/octave
at a limiter amplifier, then passed through a splatter filter
section in IC22. The output signals from pin 8 pass through
the level controller (IC5, pins 4, 3) and are then applied to
the modulation circuit (D23, D46).

The N/W switch is connected to the input lines of the splat-
ter filter circuit (N/W switch; Q65) and buffer amplifier
(IC503) (N/W switch; IC-502). The N/W switches (Q65,
IC502) are controlled by the /NWC signal from Q505 to
adjust filter cut-off frequency (by Q65) and maximum fre-
quency deviation (by IC502).

• AF circuit

Speaker

AMUTE

Volume control signal

Amp.

AF mute

switch

AF mute

switch

AF power

amp.

IC7