Transmitter Circuits - Icom IC-F2610 Service Manual

The N/W switches (D501, D502) select a ceramic filter (FI3
or bypass), and the other N/W switch (Q61) adjusts the input
level of the FM IF IC (IC1, pin 8) to switch the bandwidth
depending on the NWC signal from the CPU (IC20, pin 57).
When NWC signal becomes high level, bandwidth setting is
wide.
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 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 becomes high level, the cut-off fre-
quency 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 (MAIN UNIT)
• 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 bandwidth.
When NWC signal which is applied to Q62 becomes high
level, bandwidth setting is wide.
The noise signals from the 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).
• AF CIRCUIT
IC2
AF signal
from IC1
Amp.
AF mute
IC3 switch
HPF
Q6
HFSW AMUTE
• 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
(FRONT AND MAIN UNITS)
The microphone amplifier circuit amplifies audio signals
from the microphone, within +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) via the buffer amplifier
(IC503), and are then applied to the modulation circuit
(D46).
• MICROPHONE AMPLIFIER CIRCUIT
8
/NWC 7
Splatter
6
Q65
W/N
switch
5
12
13
14
15
16
MIC
IC22
IC7
2 9
Volume control signal
4 - 2
IC502 W/N switch
/NWC
Tx dev.
4
DTCS bal.
Tone dev.
1
13
4
DTMF
21
Limiter
SIGOUT
AMOD
TX PO
12
HPF
IC5
11
Ref.
AF mute AF power
switch amp.
Q7 IC9
SP
3
Modulator
10
circuit
9
2
Ref.
Beep
14
osc.
SQL
22
11
Speaker