Icom IC-756PRO III Service Manual page 11

3-2-4 SPEECH COMPRESSOR CIRCUIT
(DSP-A BOARD)
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
shifted signal from the level converter (IC2051) is applied
to the DSP IC (IC2001) and compressed at the DSP IC to
obtain an average audio level.
At the same time, the compressed signals are modulated
at the DSP IC and applied to the level converter (IC2052).
3-2-5 IF AMPLIFIER AND MIXER CIRCUITS
(MAIN-A AND RF-B UNITS)
The modulated 3rd IF signal from the DSP-A board (DTIF:
36 kHz) is applied to the 3rd mixer circuit (MAIN-A unit;
IC221, pin 1). The applied 3rd IF signal is mixed with the
3rd LO signal from the DDS circuit (PLL unit; IC701) to pro-
duce a 455 kHz 2nd IF signal.
The 2nd IF signal is output from (MAIN-A; IC221, pin 6)
and amplified at the IF amplifier (MAIN-A unit; Q241). The
amplified signal is passed through the ceramic bandpass
filter (MAIN-A unit; FI131: FM/AM modes, FI133: other
modes) for unwanted signals are suppressed. The filtered
2nd IF signal is amplified at IF amplifier (MAIN-A unit;
Q261) and applied to the 2nd mixer circuit on the RF-B unit
via J101 (MAIN-A unit).
The 2nd IF signal is mixed with the 64 MHz 2nd LO signal,
coming from the PLL unit, at the 2nd mixer circuit (RF-B
unit; D1852) to obtain a 64.455 MHz 1st IF signal. The 1st
IF signal is passed through a MCFs (RF-B unit; FI1701)
to cut-off the undesired signals then amplified at the IF
amplifier (RF-B unit; Q1551) via the T/R switch (RF-B unit;
D1551). The amplified 1st IF signal is applied to the 1st IF
mixer circuit (RF-B unit; D1451).
The operating (transmitting) frequency is produced at the
1st IF mixer circuit (RF-B unit; D1451) by mixing the 1st IF
and 1st LO signals. The mixed signal is then applied to the
RF circuit.
• Transmitter construction
IC3002
13
14
AMP
MIC
12
11
IC451
"MISL" signal
MAIN-A UNIT
IC3005x
12
14
VCA
DTAF
13
11
"MOSL" signal
D1852
64.455 kHz
2nd LO
(64.00 MHz)
3-2-6 RF CIRCUIT (RF-B AND PA UNITS)
The RF circuit amplifies operating (transmitting) frequency
to obtain 100 W of RF output.
The signal from the 1st IF mixer (RF-B unit; D451) is
passed through the low-pass filter (RF-B unit; L1402,
L1403, C1405–C1409) and amplified at the RF amplifier
(RF-B unit; IC1401). The amplified signal is amplified again
at the wide-band YGR amplifier (RF-B unit, IC201) after
passing through one of 10 bandpass (Refer to page 3-1
for bandpass filters used) and high-pass filters, and is then
applied to the PA unit via J201 (RF-B unit).
The signal applied from the RF-B unit is amplified at the
pre-drive (Q1), drive (Q2, Q3) and power amplifiers (Q4,
Q5) in sequence to obtain a stable 100 W of RF output
power. The amplified signal is applied to one of 8 low-pass
filters in the FILTER-A unit.
3-2-7 LOW-PASS FILTER CIRCUIT (FILTER-A UNIT)
The low-pass filter circuit contains 8 Chebyschev low-pass
filters to suppress the higher harmonic components.
The amplified signal from the PA unit is applied to one of
8 low-pass filters, which is selected by the I/O expander
(CTRL-A unit, IC11) via the buffer amplifier (CTRL-A unit;
IC12).
The filtered signal is then applied to one of 2 antenna con-
nectors via the CTRL-A unit.
3-2-8 ALC CIRCUIT (MAIN-A UNIT)
The ALC (Automatic Level Control) circuit controls the gain
of IF amplifiers in order for the transceiver to output a con-
stant RF power set by the [RF POWER] control even when
the supplied voltage shifts, and etc.
The RF power level is detected at one of the APC detector
circuits (CTRL-A unit; D2) to be converted into DC voltage
and applied to the MAIN-A unit as the FORV signal.
The FORV signal from the CTRL-A unit is applied to the
comparator (IC551b, pins 6, 7). The POCV signal, con-
trolled by the [RF POWER] control via the I/O expander
(IC3751, pin 5), is also applied to the other input (IC551b,
pin 5) for reference. The compared signal is output from
pin 7 and applied to the IF amplifiers in the MAIN-A (Q261)
and RF-B (Q1551) units to control amplifying gain.
36 kHz IF
IC221
DSP-A 455 kHz
DTIF
board
3rd LO
(491 kHz)
FI1701
D1451
Crystal
filter
1st LO
(64.485–
124.455 MHz)
3 - 5
FI131
FM/AM
modes
Ceramic
filter
FI133
other
modes
Ceramic
filter
LPF BPFs
RF-B UNIT
PAIN
PA UNIT