Other Circuits - Icom IC-F1721/D Service Manual

4-5 OTHER CIRCUITS

4-5-1 COMPANDER CIRCUIT (MAIN UNIT)
IC-F1700/F1800 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) IN CASE OF 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 fi lter
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) IN CASE OF 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 BLOCK DIAGRAM
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 CIRCUITS
RECEIVED SIGNAL
From FM IF IC
AMP
(IC12, pin 11)
Q3
To FM/PM SW
DA SW
(IC43, pin 11)
IC42
Pre-
emphasis
TX/RX
HPF
MSK
Modulator
MSK MSK
BPF Demodulator
BPF
A/D
AMP
IC15
IC16
FI3
DIGITAL
LPF
AMP
CPU
IC13
IC13
IC20
4-5-2 DIGITAL MODE CIRCUIT (IC-F1721D/F1821D only)
(1) IN CASE OF RECEIVING
Output signal from the FM IF IC (IC12, pin 11) is applied to
the IF amplifi er (Q3), and is then amplifi ed at the IF amplifi er
(IC15, pins, 1, 4) after pass through the bandpass fi lter (FI3).
The amplifi ed signal is applied to the DSP IC (IC41) via the
A/D converter (IC16, pin 3). The signal is processed at the
DSP IC (IC41) and is then applied to the LINER CODEC IC
(IC18, pin 16) to convert into an analog audio signal.
Output signal from the LINER CODEC IC (IC18) is applied
to the base band IC (IC2, pin 23) same as analog received
signal via the DA switch (IC42, pins 1, 15).
(2) IN CASE OF TRANSMITTING
Output signal from the base band IC (IC2, pin 7) is ampli-
fi ed at the MOD amplifi er (IC44) and is then applied to the
CODEC IC to convert to into a digital signal. The signal is
processed at the DSP IC (IC41) and the DIGITAL CPU (IC20),
and then applied to the low-pass fi lter (IC13, pins 1–3) after
amplifi ed at IC13 (IC13, pins 5, 7).
The signal is applied to the FM/PM switch (IC43, pins,
12–14) same as an analog transmit signal via the DA switch
(IC42, pins 13, 14).
Limiter
Scrambler/
De-scrambler
De-
emphasis
DA SW
LINER
DSP
CODEC
IC
IC
IC41 IC18
4 - 5
7 MOD
Splatter VR2 SMF
18
Expander
VR4
19
20 SIGNAL
RXA2
To AF amplifier
ciruit
BASE
IC42
BAND
IC
From
MIC amplifier
AMP
(FRONT UNIT; IC15)
IC44
IC2
TRANSMIT SIGNAL