Other Circuits - Icom IC-F2721 Service Manual

4-5 OTHER CIRCUITS

4-5-1 COMPANDER CIRCUIT (MAIN UNIT)
IC-F2700/2800 series have compander circuit which can
improve S/N ratio and become wide dynamic range. The
circuit is composed in the base band IC (IC2).
(1) WHILE RECEIVING
The demodulated AF signals from the FM IF IC (IC12, pin
9) are applied to the amplifi er section in base band IC (IC2,
pin 23), and then pass through the low-pass and high-pass
filter section to suppress unwanted signals. The filtered
signals pass through (or bypass) scrambler section, and are
then applied to the expander circuit to expand AF signals.
The output signals from the base band IC (IC2, pin20)
is applied to the AF amplifier circuit after amplified at the
amplifi er section.
(2) WHILE TRANSMITTING
The audio signals from the microphone are applied to the
base band IC (IC2, pin 3) via microphone amplifi er (FRONT
UNIT; IC5). The signals are amplifi ed at the amplifi er section,
and are then applied to the compressor circuit to compress
the audio signals. The signals pass through (or bypass)
scrambler section, and are then applied to the limiter section
after being passed through the high-pass fi lter.
The filtered signals pass through the splatter filter section,
and are then applied to the modulation circuit (D12) via the
FM/PM switch (IC43, pins12–14) and D/A converter (IC8,
pins 2, 3).
• BASE BAND IC DIAGRAM (IC2: AK2346-E2)
VR1 Com-
TXIN 3
(HPF) pressor
TXA1
VR3 RX
RXIN 23
(HPF) LPF
RXA1
SDEC 21
MTDT 10
MTCK 9
MDIR 14
MSCK 13
Control
MDIO 11
Register
MRDF 12
• DIGITAL MODE CIRCUIT
RECEIVED SIGNAL
From FM IF IC
AMP
(IC12)
Q3
To FM/PM SW
MIC SW
(IC43)
IC42
Pre-
emphasis
TX/RX
HPF
MSK
Modulator
MSK MSK
BPF Demodulator
BPF
A/D
AMP
IC15
IC16
FI3
LPF
IC13
4-5-2 DIGITAL MODE CIRCUIT (IC-F2721D/F2821D only)
(1) WHILE RECEIVING
A portion of the 2nd IF signal from the limiter amplifi er sec-
tion in the FM IF IC (IC12) output from pin 11 and is ap-
plied to the IF amplifi er (Q3). The amplifi ed signal is passed
through a ceramic bandpass fi lter (FI3) to suppress hetero-
dyne noise, and amplified again at the digital IF amplifier
(IC15, pins, 1, 4). The amplifi ed 2nd IF signal is applied to
the A/D converter (IC16) to be converted into digital IF sig-
nal, then applied to the DSP IC (IC41). The DSP IC converts
the digital IF signal into the digital audio signal.
The digital audio signal from DSP IC are converted into ana-
log audio signals at the LINER CODEC IC (IC18) and output
from pin 16.
The audio signals are applied to the base band IC (IC2,
pin 23) after being pass through the DIG/ANA switch (IC42,
pins 1, 15).
(2) WHILE TRANSMITTING
The microphone signal from the base band IC (IC2, pin 7)
are amplifi ed at the MOD amplifi er (IC44, pins 3, 4) and are
then applied to the LINER CODEC IC (IC18, pin 2) to con-
vert into the digital audio signal.
The digital audio signal is processed by DSP IC (IC41), and
applied to the LINER CODEC IC (IC18) again.
The signal from the LINER CODEC IC (IC18, pin 15) is
passed through the low-pass fi lter (IC13, pins 1, 3, 6, 7) and
then applied to the PM/FM switch (IC43, pins 12–14) after
pass through the microphone switch (IC42 pins 13, 14).
Limiter
Scrambler/
De-scrambler
De-
emphasis
IC41
DSP DIG/ANA
LINER
IC
CODEC
IC
IC42
IC18
LINER
CODEC
AMP
IC
IC44
IC18
4 - 5
Splatter VR2 SMF
Expander
VR4
RXA2
To AF amplifier circuit
SW
BASE
BAND
From
IC
MIC amplifier
(FRONT UNIT; IC15)
IC2
TRANSMIT SIGNAL
7 MOD
18
19
20 SIGNAL