Circuit Description; Receiver Circuits; Rf Circuit (Pa And Main Units); Preamplifier Circuit (Main Unit) - Icom IC-703 Service Manual

SECTION 3

3-1 RECEIVER CIRCUITS

3-1-1 RF CIRCUIT (PA AND MAIN UNITS)

HF/50 MHz RF filters pass only the desired band signals
and suppress any undesired band signals. The HF/50 MHz
RF circuit has 7 low-pass filters and 6 high-pass filters for
specified band use.
HF/50 MHz RF signals from the [ANT] connector, pass
through one of 7 low-pass filters as below, the transmit/
receive switching relay (PA unit; RL250) and low-pass filter
(PA unit; L300, L301, C304–C308), and are then applied to
the MAIN unit via J300 (PA unit).
• Used RF low-pass filter (MAIN unit)
Frequency Control Entrance
(MHz) signal coil
0.03–2 MHz F1 RL401
2–4 MHz F2 RL421
4–8 MHz F3 RL441
8–15 MHz F4 RL461
The signals from the PA unit are applied to or bypass the 20
dB attenuator (MAIN unit; R2, R3). The signals pass through
the high-pass filter (MAIN unit; L301–L303, C301–C303) to
suppress strong signals above 1.6 MHz and are then
applied to the HPF section.
(1) 0.03–2 MHz and 30–40 MHz
The signals pass through a low-pass filter (L304, L305,
C306–C310), and then applied to the preamplifier circuit on
the MAIN unit.
(2) 2–30 MHz
The signals from the low-pass filter (L304, L305,
C306–C310) are applied to one of 5 high-pass filters as at
right above and are then applied to the preamplifier circuit
on the MAIN unit.
(3) 40–60 MHz
The signals pass through the high-pass filter (L502–L504,
C501–C505) and the low-pass filter (L501, L506–L508
C506–C513), and are then applied to the preamplifier circuit
on the MAIN unit.
• 2ND IF AND DEMODULATOR CIRCUIT
PA UNIT MAIN UNIT
[ANT]
0.03—60 MHz
LPFs
HPFs

CIRCUIT DESCRIPTION

Frequency Control Entrance
(MHz) signal coil
15–22 MHz F5 RL481
22–30 MHz F6 RL501
30–60 MHz F7 RL521
1st LO:
69.0415 MHz—
530.0115 MHz
Pre-amp.
FI851
Q651
Crystal
amp.
filter
1st mixer
64.455 MHz
D701
• Used RF high-pass filter (MAIN unit)
Frequency Control
signal
(MHz)
0.03–2 MHz BTHH
2–4 MHz B2H
4–8 MHz B3H
8–15 MHz B4H

3-1-2 PREAMPLIFIER CIRCUIT (MAIN UNIT)

The preamplifier circuit in the IC-703 has approx. 15dB gain
over a wide-band frequency range.
When the preamplifier is turned ON, the signals from the RF
circuit are applied to the preamplifier (Q651) via D622 or
D632. Amplified or bypassed (passes through the D601 and
D602 when bypassed) signals are applied to the 1st mixer
circuit (D701).
3-1-3 1ST MIXER CIRCUIT (MAIN UNIT)
The 1st mixer circuit mixes the receive signals with the 1st
LO signal to convert the receive signal frequencies to a
64.455 MHz 1st IF signal.
The signals from the preamplifier circuit, or signals which
bypass the preamplifier, are passed through a low-pass fil-
ter (L681–L683, C681–C687) and then applied to the 1st
mixer (D701).
The 1st LO signal (one of the 64.485–124.455 MHz) is
amplified at Q751, filtered by a low-pass filter (L753,
C756–C759), passed through the attenuator (R771–R773),
and then applied to the 1st mixer.
3-1-4 1ST IF CIRCUIT (MAIN UNIT)
The 1st IF circuit filters and amplifies the 1st IF signals. The
1st IF signals are applied to a Crystal Filter (FI851) to sup-
press out-of-band signals.
The 64.445 MHz 1st IF signals pass through the crystal fil-
ter (FI851). Then the filtered signals are applied to the IF
amplifier (Q881).
The AGC voltage is supplied to the Q881's gate to obtain
stable signal for AGC operation.
2nd LO:
60.0 MHz
FI1501
Ceramic NB
Det.
filter
2nd
9.0115MHz
mixer
D901
3 - 1
Entrance Frequency Control
coil (MHz) signal
D541 15–22 MHz B5H
D551 22–30 MHz B6H
D561 30–40 MHz BTHH
D571 40–60 MHz B7RH
FI1301
SSB, CW
filter
to AM demod.
FI1341
circuit (D2001)
AM, FM-N
to BFO circuit
filter
(IC1901)
to FM demod.
FM mode
circuit (IC2101)
Optional
filter
Entrance
coil
D581
D591
D541
D501