Transmitter Circuits - Icom IC-7400 Service Manual

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN 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 IC1400
(pins 2, 4), and then applied to the VCA section (IC1400, pin
7). The gain controlled signals are output from (IC1400, pin
9) and amplified at the AF amplifier (IC1401, pins 1–3) and
passed through the low-pass filters (IC1401, pins 5–9) and
analog switch (IC1310, pins 12, 14). The signals are then
applied to the DSP circuit as the DTAF signal.
The VCA section in IC1400 (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 (IC502, pin 3).
4-2-2 VOX CIRCUIT (MAIN UNIT)
The VOX (Voice-Operated-Transmission) circuit sets trans-
mitting conditions according to voice input.
When the VOX function is activated, a portion of the micro-
phone signals are amplified at the VOX amplifier (IC1280
pins 3, 1) and applied to main CPU (IC1500, pin 109) via the
VOXL line. A portion of the power amplified AF signals from
the AF power amplifier (IC1240) are amplified at the anti-
VOX amplifier (IC1121, pins 6, 7) and applied to the main
CPU (IC1500, pin 110) via the AVXL line. Then the main CPU
compares these and controls the transmitter circuit.
4-2-3 DSP TRANSMITTER CIRCUIT (DSP UNIT)
The DSP (Digital Signal Processor) circuit enables PSN
(Phase Shift Network)/Low Power/Phase modulation, trans-
mitter monitor, side tone, and etc.
The microphone audio signals from the MAIN unit via the
DTAF line are amplified at the differential amplifiers
(IC601a/b), and are then applied to the A/D converter sec-
tion in the CODEC IC (IC501). At the same time, the con-
verted signals are level-shifted 5 V to 3.3 V in the IC (IC501).
The level-shifted signal are applied to the DSP IC (IC301)
and modulated at the DSP IC to produce the 36 kHz trans-
mitter IF signal.
• Transmitter construction
IC1400
MIC
AMP VCA
MAIN UNIT
IC1310
13
DSP
12 DTAF
unit
14
9
"MODS" signal
FI231b/a
D261
64.455 kHz Crystal
Bandwidth
2nd LO
15 kHz
(64.00 MHz)
The modulated IF signal from the DSP IC is applied to the
D/A converter section in the CODEC IC (IC501) to convert
into the analog IF signal. Also the signal is level-shifted 3.3
V to 5V at the level converter section in the IC (IC501).
The level-shifted IF signal is passed through the active filter
(IC701b), and then applied to the MAIN unit via J901 (pin
22) as the DTIF signal.
4-2-4 SPEECH COMPRESSOR CIRCUIT
(DSP UNIT)
The speech compressor compresses the transmitter audio
input signals to increase the average output level (average
talk power).
When the speech compressor function is ON, the level-shift-
ed signal from the CODEC IC (IC501) is applied to the DSP
IC (IC301) 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 D/A converter section in the
CODEC IC (IC501).
4-2-5 IF AMPLIFIER AND MIXER CIRCUITS
(MAIN AND RF UNITS)
The modulated 3rd IF signal from the DSP unit (DTIF: 36
kHz) is amplified at the 3rd IF amplifier (MAIN unit; IC1120a)
and passed through the low-pass filters (MAIN unit;
IC1120b/c). The filtered signal is applied to the 3rd mixer cir-
cuit (MAIN unit; IC400, pin 1). The applied 3rd IF signal is
mixed with the 3rd LO signal from the DDS circuit (RF unit;
IC1601) to produce a 455 kHz 2nd IF signal.
The 2nd IF signal is output from pin 6 and passed through
the ceramic bandpass filter (MAIN unit; FI381) for unwanted
signals are suppressed. The filtered 2nd IF signal is ampli-
fied at the 2nd IF amplifier (MAIN unit; Q330). The amplified
signal is applied to the 2nd mixer circuit on the RF unit via
J250.
The 2nd IF signal is mixed with the 64 MHz 2nd LO signal,
coming from the PLL circuit, at the 2nd mixer circuit (RF unit;
D261) to obtain a 64.455 MHz 1st IF signal. The 1st IF sig-
nal is passed through the MCFs (RF unit; FI231b/a) to cut-
off the undesired signals, and is then amplified at the 1st IF
amplifier (RF unit; Q271) via the T/R switch (RF unit; D251).
The amplified 1st IF signal is applied to the 1st IF mixer cir-
cuit (RF unit; D271) via the attenuator (RF unit; D272) and
low-pass filter circuit.
36 kHz IF
IC400
DTIF 455 kHz
3rd LO
(491 kHz)
D271
LPF
filter
1st LO
4 - 4
FI381
Ceramic
filter
BPFs
RF UNIT