Icom IC-F610 Service Manual page 8

The 2nd IF signal from the 2nd mixer section (IC1, pin 3)
passes through a ceramic filter (FI2) to remove unwanted
heterodyned frequencies. It is then amplified at the limiter
amplifier section (IC1, pin 5) and applied to the quadrature
detector section (IC1, pins 10, 11) to demodulate the AF sig-
nals.
The AF signals are output from pin 9 (IC1) and are then ap-
plied to the AF amplifier circuit.
4-1-5 AF AMPLIFIER CIRCUIT (MAIN UNIT)
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 passed
through the high-pass filter (IC21, pins 5, 7) and then applied
to the de-emphasis section of the compander IC (IC14, pin 3)
with frequency characteristics of –6 dB/octave. The signals
are passed through the low-pass filter, high-pass filter, ex-
pander sections in the compander IC (IC14). The signal out-
put from pin 38 (IC14) and then applied to the D/A converter
(IC6, pins 1, 2).
The output AF signals from the D/A converter (IC6, pin 2) are
applied to the AF amplifier (IC18, pins 3, 4) and AF power
amplifier (IC8, pins 1, 4).
The power amplified AF signals are applied to the internal
speaker that is connected to J5 via [EXT SP] jack (J4).
• AF AND MIC AMPLIFIER CIRCUIT
AF signal
from FM IF IC (IC1, pin 9)
IC15
PM/FM switch
IC21
IC6
21
D/A
LPF
converter
"CTCSS/DTCS" signal from
D/A conveter IC (IC6, pin 11)
"TONE" signal from Q38
3
38
Compander IC
(IC14)
6
12
D18
22
FM mod.
4-1-6 RECEIVER MUTE CIRCUITS (MAIN UNIT)
• 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.
Some noise components in the AF signals from the FM IF IC
(IC1, pin 9) are passed through the D/A converter (IC6, pins
23, 24). 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 only. The amplified noise signals
are converted into the pulse-type signals at the noise detec-
tor section. The detected signals output from pin 13 (NOIS)
via the noise comparator section.
The NOIS signal from the FM IF IC (IC1, pin 13) is applied
to the CPU (IC20, pin 37). Then the CPU analyzes the noise
condition and outputs the AF mute signal as "AFON" from
the pin 18 to the AF power controller (Q39, Q40, D30, D31).
• 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 the noise squelch is open.
A portion of the AF signals from the FM IF IC (IC1, pin 9)
passes through the low-pass filters (IC5 pins, 1, 2, 5, 7) to
remove AF (voice) signals and is applied to the CTCSS or
DTCS decoder inside the CPU (IC20, pin 46) as the "CDEC"
signal. The CPU outputs the AF mute signal as "AFON" from
the pin 18 to the AF power controller (Q39, Q40, D30, D31).
IC6
D/A
1 2 3
converter
IC6
9
10
D/A
converter
to TX VCO circuit
(Q14, D17)
4 - 2
IC18
IC8
AF
4 1 4
AMP
AMP
IC2
3
4
AMP
FRONT UNIT
Speaker
Microphone