Transmitter Circuits - Icom IC-756PRO Service Manual

3-2 TRANSMITTER CIRCUITS

3-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN UNIT)
The microphone amplifier circuit amplifies microphone audio
signals to a level needed for the DSP.
Audio signals from the [MIC] connector (MIC board; J1, pin
1) are amplified at the audio amplifier section in IC451 (pins
21–23) 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 14, 12) and then applied to the DSP circuit as
the DTAF signal.
The VCA section in IC451 (pins 7–9) 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 UNIT)
The VOX (Voice-Operated Transmission) circuit sets trans-
mitting conditions according to voice input.
A portion of the amplified audio signals from the AF amplifi-
er section in IC451 are again amplified at the VOX amplifier
section IC451 (pin 9), also gain contolloed signals at the
VCA section (pin 9) are amplified at 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 6, 1) as the VOXL signal.
The VOGV signal is applied to the VCA section in IC3003
(pin 7–9) 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 BOARD)
The microphone audio signals from the MAIN unit via the
DTAF line are passed through the analog switch (IC2201,
pins 4 and 3 or 5) and applied to the each modulation cir-
cuits.
(1) When SSB mode
The audio signals from the analog switch (IC2201, pin 5) are
amplified at the limitter amplifier (IC2281b) and applied to
the low-pass filter (IC2281d/c) to limit the transmit passband
width.
• Transmitter construction
MIC
AMP VCA
IC451
"MOSL" signal
MAIN UNIT
IC3005X
12
14
DSP
DTAF
13
board
11
D741
64.455 kHz
Bandwidth
2nd LO
(64.00 MHz)
The filtered signals are then applied to the differential ampli-
fiers (IC2301a/b) via the analog switch (IC2201) and T/R
switch (IC2291).
(2) When FM/AM modes
The audio signals from the analog switch (IC2201, pin 3) are
applied to the deviation adjustment pots (R2227: FM mode,
R2229: AM mode) via the limitter amplifier, pre-emphasis cir-
cuit (only FM mode) and splatter filter consist of IC2211.
The level adjusted signals are applied to the differential
amplifiers (IC2301a/b) after being passed through the ana-
log switch (IC2201) and T/R switch (IC2291). When AM
mode the pre-emphasis circuit is cancelled by Q2201,
Q2202, Q2211.
The amplified signals at the differential amplifiers
(IC2301a/b) are applied to the A/D converter (IC2321). The
coverted signals are level shifted 5V to 3.3 V at the level
converter (IC2051).
The level shifted signal is applied to the DSP IC (IC2001)
and modulated at the DSP IC to produce the 36 kHz trans-
mit IF signal. The modulated IF signal from the DSP IC is
level shifted 3.3 V to 5V at 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 filter
(IC2371a), buffer amplifier (IC2371b), analog switch
(IC2372, pins 14, 12) then applied to the low-pass filter
(IC2381c/d). The filtered signal is applied to the MAIN unit
via J2001 (pin 28) as the DTIF signal.
A portion of the filtered signal from the low-pass filter
(IC2381c/d) is amplified at the IF amplifier (IC2381b) and
applied to the transmit monitor circuit for the monitor func-
tion.
3-2-4 SPEECH COMPRESSOR CIRCUIT
(DSP BOARD)
The speech compressor compresses the transmitter audio
input signals to increase the average output level (average
talk power).
When the [COMP] switch is ON, the level shifted signal from
the level converter (IC2051) is applied to the DSP IC
(IC2001) and compressed at the DSP IC to obtain an aver-
age audio level.
At the same time, the compressed signals are modulated at
the DSP IC and applied to the level converter (IC2052).
36 kHz IF
IC221
DTIF 455 kHz
3rd LO
(491 kHz)
FI711
D771
Crystal
filter
15 kHz
1st LO
3 - 4
FI131
FM/AM
modes Ceramic
filter
FI133
other
modes Ceramic
filter
LPF BPFs
RF UNIT