Receiver Circuit - Icom IC-756PRO III Service Manual

SECTION 3

3-1 RECEIVER CIRCUIT

3-1-1 RF SWITCHING CIRCUIT
(CTRL-A AND RF-B UNITS)
The RF switching circuit leads receive signals to bandpass
fi lters from an antenna connector while receiving. However,
the circuit leads the signal from the RF power amplifier to
the antenna connector while transmitting.
RF signals from [ANT 1] or [ANT 2] pass through the an-
tenna selector (RL3), transmit/receive switching relays (RL1,
RL2, RL4), and low-pass fi lter (L27, L28, C63–C66, C105),
and are then applied to the RF-B unit via J101 (RF-B unit).
The signals from the CTRL-A unit either bypass or pass
through the 6 dB (RF-B unit, R102, R106, R111, RL102)
and/or 12 dB (RF-B unit, R112, R113, R114, RL103)
attenuators via the antenna selector (RL101). By selecting
the attenuators, 0 (bypass), 6, 12 and 18 dB attenuations
are obtained. The signals are then applied to the RF fi lters.
When the [RX ANT] is selected, the RF signals are passed
through the low-pass filter (RF-B unit, L101, L102, C101
–C105), then applied to the antenna selector (RF-B unit,
RL101).
3-1-2 RF BANDPASS FILTER CIRCUIT
(RF-B UNIT AND BPF-A BOARD)
RF bandpass fi lters pass only the desired band signals and
suppress any undesired band signals. The RF circuit has 11
bandpass fi lters and 1 low-pass fi lter.
(1) 0.03–1.6 MHz (RF-B UNIT)
The signals pass through the low-pass filter (L801–L802,
C802, C805–C807), attenuator (R801–R803), and are then
applied to the RF amplifi ers (Q1001, Q1002).
(2) 1.6–60 MHz (RF-B UNIT AND BPF-A BOARD)
The signals pass through the band switch (D104) and high-
pass filter (L251–L253, C251, C252, C271–C274) to sup-
press excessively strong signals below 1.6 MHz. The fi ltered
signals are applied to one of 11 bandpass fi lters on the table
at right above, and then applied to or bypassed the pre-
amplifi er circuit.
• Receiver construction
1st mixer B
Q1003–Q1006
LPF or
1st LO B
BPF
0.03–60.0 MHz
1st LO A
CIRCUIT DESCRIPTION
2nd mixer
FI1701
D1752
Crystal
filter
64.455 MHz
2nd LO
1st mixer A
64.0 MHz
Q1203–Q1206
RF-B UNIT
• Used RF fi lter
Control
Band
signal
0.03–1.6 MHz B0
1.6–2 MHz B1
2–3 MHz B2
3–4 MHz B3
4–6 MHz B4
6–8 MHz B5
8–11 MHz B6
*: On the BPF-A board
3-1-3 PRE-AMPLIFIER CIRCUITS (PRIAMP BOARD)
The IC-756PROIII has 2 gain levels of pre-amplifi er circuits.
One has 10 dB gain for the 1.8–21 MHz bands and the other
one has 16 dB gain for the upper 24 MHz bands.
When the [P.AMP] switch is set to [P.AMP 1] or [P.AMP
2], the signals are applied to the pre-amplifier 1 (Q4201,
Q4202) or pre-amplifi er 2 (Q4302) circuit, respectively. Pre-
amplifi ed or bypassed signals are applied to the RF amplifi er
circuits (RF-B unit; Q1001, Q1002 or Q1201, Q1202).
3-1-4 RF AMPLIFIER AND 1ST MIXER CIRCUITS
(RF-B UNIT)
The 1st mixer circuit mixes the receive signals with the 1st
LO signal to convert the receive signal frequencies into a
64.455 MHz 1st IF signal. The IC-756PROIII has two 1st
mixer circuits for the dualwatch function.
The signals from the pre-amplifier circuit, or signals which
bypass the pre-amplifi ers, are divided at L902, L903. Each
signal is applied to a 60 MHz cut-off low-pass fi lter, RF am-
plifi er (Q1001, Q1002 for sub readout or Q1201, Q1202 for
main readout) and then to a 1st mixer (Q1003–Q1006 sub
readout or Q1203–Q1206 for main readout) to convert the
frequency into the 64.455 MHz 1st IF signal.
Each 1st LO signal (64.4850–124.4550 MHz) from the PLL
unit via J1101 or J1301. The LO signals are amplifi ed at the
LO amplifi er (Q1101; sub or Q1301; main), fi ltered by a low-
pass fi lter, and then applied to each 1st mixer.
3rd mixer
FI111
IC151
36 kHz
Ceramic
filter
455 kHz
3rd LO
491 kHz
MAIN-A UNIT
3 - 1
Input Control
Band
diode signal
D801 11–15 MHz B7
*D3201 15–22 MHz B8
*D3301 22–30 MHz B9
*D3401 30–50 MHz B10W
*D3501 50–54 MHz B10
*D3601 54–60 MHz B10W
D501
DSP-A
to squelch gate
board
(IC301)
Input
diode
D551
D602
D651
D701
D751
D701