Receiver Mute Circuits (Logic Unit); Transmitter Circuits; Af Amplifier Circuit - Icom IC-FR4000 Service Manual

3-1-6 RECEIVER MUTE CIRCUITS (LOGIC 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
(RX unit; IC2, pin 9) are passed through the SQL level con-
troller (VR unit; R2). The level controlled signals are applied
to the active filter section in the FM IF IC (RX unit; IC2, pin
8). Noise components about 10 kHz are amplified and out-
put from pin 7 (RX unit; IC2).
The filtered signals are converted to the pulse-type signals
at the noise detector section and output from pin 13 (RX
unit; IC2).
The NDET signal from the FM IF IC (RX unit; IC2) is applied
to the CPU (IC33, pin 40). The CPU analyzes the noise con-
dition and controls the AF mute signal via "AFMUTE1" line
(IC40, pin 4) to the AF mute switch (IC16).
• 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).
The CTCSS signal passes through the low-pass filter circuit
(IC8) and is then applied to the CTCSS decoder IC (IC29).
The detected signal is applied to the CPU (IC 33) via the ser-
ial signal line.
• TONE SIGNALS CIRCUITS (LOGIC UNIT)
DISC signal
from RX unit
AMP
IC9
• AF AMPLIFIER CIRCUITS (LOGIC UNIT)
MIC
From the FRONT
unit
From the REMOTE E_MOD
connector
From the telephone
TELAFO
circuit
From the ACC
E_MODE2
connector
CTCSS
AMP
LPF
IC8
DTCS
LPF
AMP
IC12
2/5TONE
LPF
DTMF DTMF DTMF
SW DECODER
IC30
IC31
SW-D
AMP
SW-C
AMP
SW-B
AMP
SW-A
AMP
IC13
MIC MUTE
The DTCS signal passes through the low-pass filter circuit
(IC12), and is then applied to the DTCS decoder which is
inside the CPU (IC33, pin 52) via the "DTCSI" line.
The 2/5TONE signals are pass through the low-pass filter
circuit (IC12), and are then applied to the 2/5TONE decoder
which is inside the CPU (IC33 pin 51) via "25TI" line.
The DTMF signal passes through the DTMF switch IC
(IC30), and is then applied to the DTMF decoder which is
inside the CPU (IC33, pins 82, 85, 86).
The DTMF switch (IC30) selects the signal from telephone
line or RX unit.

3-2 TRANSMITTER CIRCUITS

3-2-1 AF AMPLIFIER CIRCUIT (LOGIC UNIT)
• IN CASE OF THE AF SIGNAL FROM THE MIC JACK
The AF signals (MIC) from the MIC jack (FRONT unit; J7)
are amplified at the AF amplifier (IC1). The amplified signals
are mixed with the "E_MOD", "E_MOD2" and "TELAFO" sig-
nals from IC1. The mixed signals pass though the high-pass
filter (IC2) via the pre-emphasis circuit (IC2).
The filtered signals are applied to the limiter amplifier, and
are then mixed with the DTMF signal from the CPU (IC33,
pin 43) via TX-signal filter (IC3). The mixed signal applied to
the splatter filter (IC4).
To AF amplifier
CTCSS
DETECTOR
IC29
CPU; IC33
DATA BUS
PRE
HPF
MIX
EMP
IC2
RPT AF signal
AFATTO
RPT AF
from IC25
MUTE
From the REMOTE
E_MOD3
connector
25TONE
BEEP
CW
DTMF
From CPU (IC33)
3 - 2
SIG
SPLAT
LIMIT
MIX
FIL
IC4
IC3
IC3
IC14
AMP
IC26
LPF
To TX unit