Transmitter Circuits - Icom IC-756PRO III Service Manual

3-2 TRANSMITTER CIRCUITS

3-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN-A UNIT)
The microphone amplifier circuit amplifies microphone
audio signals to a level needed for the DSP circuit.
Audio signals from the [MIC] connector (MIC board; J1,
pin 1) are amplified at the audio amplifier section in IC451
(pins 21, 22) via the analog switch (IC3002, pins 12, 14),
then applied to the buffer amplifier section (IC451, pin 5)
and VCA section. The gain controlled signals are output
from (IC451, pin 9) and passed through the analog switch
(IC3005, pins 12, 14) and then applied to the DSP circuit
via J201 (pin 15) as the DTAF signal.
The VCA section in IC451 (pin 9) controls microphone
input gain according to the [MIC GAIN] control level using
the MIGV signal coming from the main CPU via the I/O
expander (IC3751, pin 4).
3-2-2 VOX CIRCUIT (MAIN-A UNIT)
The VOX (Voice-Operated Transmission) circuit sets trans-
mitting conditions according to voice input.
A portion of the audio signals from the VCA section (IC451,
pin 9) is applied to the AF amplifier (IC3004b, pins 6, 7),
and then applied to the main CPU (IC3501, pin 106) after
passing through the analog switch (IC362, pins 1, 6) as the
VOXL signal.
The VOGV signal is applied to the VCA section in IC3003
(pin 8) from the main CPU via the I/O expander (IC3751,
pin 9) to adjust VOX actionable sensitivity. This is controlled
by the VOX gain set in the VOX SET mode.
3-2-3 DSP TRANSMITTER CIRCUIT
(DSP-A BOARD)
The microphone audio signals from the MAIN-A unit via the
DTAF line are applied to the analog switch (IC2201, pin 4)
and output from pin 3 or 5 to the each modulation circuits.
• DSP Transmitter circuit
IC2201z
9
5
AF
DTAF
4
signal
3
Mode switch
IC2321
converter
MAIN-A unit DSP-A board
"MODS" signal
IC2281b IC2281c/d
SSB
mode
Limitter LPF
IC2211a IC2211b
FM/AM
mode
Limitter
LPF
IC2051
IC2001
A/D Level
DSP IC
converter
(1) When SSB mode
The audio signals from the analog switch (IC2201, pin 5)
are amplified at the limiter amplifier (IC2281b) and applied
to the low-pass filter (IC2281c/d) to limit the transmit
passband width.
The filtered signals are then applied to the differential
amplifiers (IC2301a/b) via the analog switch (IC2201, pins
12, 14) and T/R switch (IC2291, pins 1, 7).
(2) When FM/AM modes
The audio signals from the analog switch (IC2201, pin 3)
are applied to the modulation adjustment pots (R2227:
FM mode, R2229: AM mode) via the limiter amplifier, pre-
emphasis circuit (only FM mode) and splatter filter consist
of IC2211. The level adjusted signals are applied to the dif-
ferential amplifiers (IC2301a/b) after being passed through
the analog switch (IC2201, pins 1, 2, 13–15) and T/R
switch (IC2291 pins 1, 7). The pre-emphasis circuit is can-
celled by Q2201, Q2202, Q2211 on AM mode.
The amplified signals at the differential amplifiers (IC2301a/
b) are applied to the A/D converter (IC2321 pins 4, 5). The
converted signals are changed from 5 V level signals to 3.3
V level signals in the level converter (IC2051).
The converted signals are applied to the DSP IC (IC2001)
and modulated at the DSP IC to produce the 36 kHz trans-
mitter IF signal. The modulated IF signal from the DSP IC
is changed from a 3.3 V signal to a 5 V signal in the level
converter (IC2052), and is applied to the D/A converter
(IC2351) to convert into the analog IF signal.
The converted IF signal is passed through the active fil-
ter (IC2371a), buffer amplifier (IC2371b), analog switch
(IC2372, pins 12, 14) then applied to the low-pass filter
(IC2381c/d). The filtered signal is applied to the MAIN-A
unit via J2001 (pin 29) as the DTIF signal.
When SSB or RTTY mode, a portion of the filtered signal
from the low-pass filter (IC2381c/d) is amplified at the IF
and buffer amplifiers (IC2381a/b) and is applied to the
transmit monitor circuit for the monitor function.
IC2291
5
6
IC2201x
11
12
1
14 7
13
T/R switch
Mode switch
IC2372x
IC2052 IC2351
Level D/A
14
converter converter
"TXS" signal
3 - 4
"TXS" signal
IC2301a/b
Differential
converter
13
IC2381d/c
DTIF
12
LPF
11
MAIN-A unit
36 kHz IF