Transmitter Circuits; Tx Af Circuits; Digital Converter Circuits; Modulation Circuits - Icom ID-880H Service Manual

4-2 TRANSMITTER CIRCUITS

TX AF CIRCUITS

MIC signals from the connected microphone are passed
through the HPF (Q87), and amplified by the MIC AMP
(IC28). The amplified MIC signals are passed through the
MIC gain SW (Q88) which selects the MIC sensitivity from
"High" or "Low," and the MIC mute SW (IC30), then passed
through or bypassed the ALC AMP (IC32) via the AF SWs
(IC29 and IC52).
In the DV mode, the MIC signals are applied to the ALC
AMP (IC32) which automatically adjusts the level of MIC
signals for digital processing,
• TX AF CIRCUITS
J2
1
2
3
Q87
Q 88
4 IC28
5
MIC MIC GAIN
MIC
6 HPF
AM P
SWITCH
7
8
From the microphone

DIGITAL CONVERTER CIRCUITS

The MIC signals from the TX AF circuits are applied to
the liner CODEC IC (IC1013) via the buffer (IC1014), and
encoded into the digital audio signal. The encoded digital
audio signal is then applied to the DSP CODEC IC (IC1006)
and converted into the AMBE signal. The AMBE signal
is applied to the modem (IC1012) via the level converter
(IC1000, 1001,1002). The modem IC converts the AMBE
signal into the analog signal, and output to the modulation
circuits via the buffer (IC1015) and the digital/analog line SW
(IC34).
• DIGITAL CONVERTER CIRCUITS
IC1013 IC1006
IC1014
From the TX AF
circuits
LINEAR
BUFF
CODEC CODEC
IC1012
MODEM
IC30 IC29
MIC
AF
MUT E SW
IC32
ALC
AM P
IC1000,1001,1002
DSP
LEVEL
CNV.
IC1015 IC34
To the modulation circuits
Digi/Ana
BUFF
line SW
The MIC signals from the AF SW (IC52) are passed through
the pre-emphasis and IDC (for amplitude-limitting) circuits
(IC48b), MIC level adjustment circuit (IC48a) and the
splatter circuit (IC48d) which cuts off the 3 kHz and higher
audio signals. The filtered MIC signals are applied to the
modulation circuits via the AF SW (IC51; pins 1, 7).
In the DV mode, the filtered MIC signals are applied to
the digital converter circuits before being applied to the
modulation circuits via the AF SW (IC51; pins 1, 6).
IC52 IC48a IC48d
IC48b
Pre-emphasis
Level SPLATTER
AF
and IDC ADJ.
SW

MODULATION CIRCUITS

The AF signals from the TX AF circuits (in FM mode) or
digital converter circuits (in DV mode) are applied to the AF
AMP (IC48c). The amplifi ed modulation signals are passed
through the D/A converter (IC8) to be adjusted its level
(=deviation), then applied to the VCO (VHF VCO: Q111,
D145–148/UHF VCO: Q73, D87, 91, 92) via the modulation
mute SWs (IC9, VHF; Q109/UHF; Q64) as the modulation
signals.
The modulated VCO oscillating signal is passed through the
buffer (VHF; Q113/UHF; Q76) and applied to the LO AMP
(IC45) via the VCO SW (VHF; D160, 175/UHF; D102).
The amplified VCO output signals are applied to the TX
amplifi er circuits.
• MODULATION CIRCUITS
IC9
IC8
From the TX AF
IC48c
circuits (FM mode)
AMP
From the digital converter
circuits (DV mode)
4 - 4
IC51 To the modulation circuits
(FM mode)
AF
SW
To the digital converter circuits
(DV mode)
DA_SEL
D160,D175
VHF VCO
Q113
Q111
VCO
D145-148 BUFF
SW
Q109
D102
MOD
MUTE
VCO
SW
Q64
UHF VCO
Q76
MOD
MOD
BUFF
MUTE
MUTE
Q73
D87,91,92
D/A
IC45 To the TX amplifer
circuits
LO
AMP