Icom IC-F2721 Service Manual page 9

A 2nd LO signal (45.9 MHz) is produced at the PLL circuit
by tripling it's reference frequency (X2: 15.3 MHz).
The 2nd IF signal from the 2nd mixer section (IC12, pin 3)
passes through the ceramic filter(s) (Wide: FI1 only, Narrow:
both FI1 and Fl2) to remove unwanted heterodyned frequen-
cies. It is then amplified at the limiter amplifier section (IC12,
pin 5) and applied to the quadrature detector section (IC12,
pins 10, 11) to demodulate the 2nd IF signal into AF signals.
The demodulated AF signals are output from pin 9 (IC12)
and applied to the base band IC (IC2).
4-1-5 AF AMPLIFIER CIRCUIT (MAIN UNIT)
The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker. This transceiver employs the base
band IC which is composed of pre-amplifier, expander,
scrambler, MSK de-modulator, etc. at the AF amplifier sec-
tion.
The AF signals from the FM IF IC (IC12, pin 9) are passed
through the DIG/ANA switch (IC42, pins 2, 15) and are then
applied to the base band IC (IC2, pin, 23). The signals are
amplified at the AF amplifier section in the base band IC (IC2,
pin 23), and are then applied to the high-pass and low-pass
filter sections of it.
The signals pass through the high-pass filter to suppress
unwanted harmonic components. The filtered signals pass
through (or bypass) scrambler and expander sections. The
signals are amplified at the amplifier section in the base
band IC (IC2).
The output signals from the base band IC (IC2, pin 20) are
passed through the RX/PA switch (IC43, pins 1, 15) and
DIG/ANA switch (IC42, pins 3, 4). The signals are applied
to the AF volume (IC8, pins 15, 16), and are then applied to
the AF amplifiers (IC34, IC38).
• AF AND MIC AMPLIFIER CIRCUIT
AF signal
from FM IF IC (IC12, pin 9)
IC42
23
2
15
Base band IC
1
IC42
IC43
14
FM/PM switch
IC1
IC8
4
D/A converter
LPF
"CTCSS/DTCS" signal from
D/A conveter (IC8, pin 10)
"TONE" signal from
D/A converter (IC8, pin 22)
IC43
20 1
15
2
(IC2)
12 7 Base band IC
3
(IC2)
13
D12
3
FM mod.
The power amplified AF signals are applied to the internal
speaker (IC-F2700 series only) that is connected to J6 via
[EXT SP] jack (J5).
4-1-6 SQUELCH 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 sig-
nals, the squelch circuit switches the AF amplifier controller.
Some noise components in the AF signals from the FM IF
IC (IC12, pin 9) are passed through the D/A converter (IC8,
pins 1, 2). The signals are applied to the active filter section
in the FM IF IC (IC12, 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. The detected signals output from pin 13 (NOIS) via
the noise comparator section.
The "NOIS" signal from the FM IF IC is applied to the Main
CPU (IC23, pin 37). Then the Main CPU analyzes the noise
condition and outputs the AF mute signal as "AFON" from
the pin 19 to the AF power controller (Q28, Q29, D34, D36).
• CTCSS AND DTCS
The tone squelch circuit detects tone 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 FM IF IC (IC12, pin 9)
pass through the low-pass filter (Q54) to remove AF (voice)
signals, and are then applied to the amplifier (Q54). The
amplified signals are applied to the CTCSS or DTCS decod-
er in the Main CPU (IC23, pin 46) via the "CDEC" line. The
Main CPU outputs the AF mute signal as "AFON" from the
pin 19 to the AF power controller (Q28, Q29, D34, D36).
IC42
IC8
3
16 15 3
4
5
IC5 IC9
3
4
5 ALC
MIC
AMP AMP
FRONT UNIT A/B
to TX VCO circuit
(Q12, D70, D71)
4 - 2
IC34
IC38
AF
4 1 4
AMP
AMP
IC8
3
6
1
7
Speaker
Microphone