Icom IC-756PRO Service Manual page 11

3-2-5 IF AMPLIFIER AND MIXER CIRCUITS
(MAIN AND RF UNITS)
The modulated 3rd IF signal from the DSP board (DTIF: 36
kHz) is applied to the 3rd mixer circuit (MAIN unit; IC221).
The applied 3rd IF signal is mixed with the 3rd LO signal
from the DDS circuit (PLL unit; IC701) to produce a 455 kHz
2nd IF signal.
The 2nd IF signal is output from pin 6 and amplified at the IF
amplifier (MAIN unit; Q241). The amplified signal is passed
through the ceramic bandpass filter (MAIN unit; FI131:
FM/AM modes, FI133: other modes) for unwanted signals
are suppressed. The filtered 2nd IF signal is ampllified at IF
amplifier (MAIN unit; Q261) and applied to the 2nd mixer cir-
cuit on the RF unit via J101.
The 2nd IF signal is mixed with the 64 MHz 2nd LO signal,
coming from the PLL unit, at the 2nd mixer circuit (RF unit;
D741) to obtain a 64.455 MHz 1st IF signal. The 1st IF sig-
nal is passed through the MCF (RF unit; FI711) to cut-off the
undesired signals then amplified at the IF amplifier (RF unit;
Q751) via the T/R switch (RF unit; D711). The amplified 1st
IF signal is applied to the 1st IF mixer circuit (RF unit; D771).
The operating (transmitting) frequency is produced at the 1st
IF mixer circuit (RF unit; D771) by mixing the 1st IF and 1st
LO signals. The mixed signal is then applied to the RF cir-
cuit.
3-2-6 RF CIRCUIT (RF AND PA UNITS)
The RF circuit amplifies operating (transmitting) frequency
to obtain 100 W of RF output.
The signal from the 1st IF mixer is passed through the low-
pass filter (RF unit; L961, L962, C961–C966) and amplified
at the RF amplifier (RF unit; IC961). The amplified signal is
again amplified at the wide-band YGR amplifier (RF unit,
IC151) after passing through one of 11 bandpass (Refer to
page 3-1 for bandpass filters used) and high-pass filters,
and is then applied to the PA unit via J151.
The signal applied from the RF 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
• DSP Transmitter circuit
IC2201Z
9
5
AF
DTAF
4
signal
3
Mode switch
IC2321
converter
MAIN unit DSP board
"MODS" signal
IC2281B IC2281C/D
SSB
mode
Limitter LPF
IC2211B
IC2211A
FM/AM
mode
Limitter
LPF
IC2051 IC2001
A/D Level
DSP IC
converter
amplified signal is applied to one of 8 low-pass filters in the
FILTER unit.
3-2-7 LOW-PASS FILTER CIRCUIT (FILTER UNIT)
The low-pass filter circuit contains 8 Chebyschev low-pass
filters to suppress the higher harmonic components.
The signal from the power amplifiers in the PA unit is applied
to one of the low-pass filters, which is selected by the I/O
expander (IC11) in the CTRL unit via the buffer-amplifier
(CTRL unit; IC12).
The filtered signal is then applied to one of 2 antenna con-
nectors via the CTRL only/and TUNER unit/s.
3-2-8 ALC CIRCUIT (MAIN 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, etc.
The RF power level is detected at one of the APC detector
circuits (CTRL unit; D2) to be converted into DC voltage and
applied to the MAIN unit as the FORV signal.
The FORV signal from the CTRL unit is applied to the com-
parator (IC551b, pin 6). The POCV signal, controlled by the
[RF POWER] control via the I/O expander (IC3751, pin 5), is
also applied to the other input (pin 5) for reference. The
compared signal is output from pin 7 and applied to the IF
amplifiers in the MAIN (Q261) and RF (Q751) units to con-
trol amplifying gain.
When the FORV signal exceeds the POCV voltage, ALC
bias voltage from the comparator controls the IF amplifiers.
This adjusts the output power to a specified level from the
[RF POWER] control until the FORV and POCV voltages are
equalized.
In AM mode, the comparator operates as an averaging ALC
amplifier. Q502 turns ON and the POCV voltage is shifted
for 40 W AM output power (maximum) through R510.
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 - 5
"TXS" signal
IC2301A/B
Differential
converter
13
IC2381D/C
DTIF
12
LPF
11
MAIN unit
36 kHz IF