Circuit Description; Receiver Circuits; Rf Switching Circuit; Used Rf Filter - Icom IC-718 Service Manual Addendum

SECTION 4

4-1 RECEIVER CIRCUITS

4-1-1 RF SWITCHING CIRCUIT

(FILTER AND MAIN UNITS)
The RF switching circuit leads receive signals to bandpass
filters from the antenna connector while receiving. While
transmitting, this circuit leads signals from the RF power
amplifier to the antenna connector. This circuit includes a 20
dB RF attenuator circuit to prevent distortion from very
strong signals.
RF signals from the antenna connector pass through the
transmit/receive switching relay (RL13), and low-pass filter
(L14, C14–C16), and are then applied to the MAIN unit via
J3 (MAIN unit; J701).
The signals from the FILTER unit are either bypassed or are
attenuated at the 20 dB attenuator (D701, R703). The sig-
nals are then applied to RF filters.
4-1-2 RF BANDPASS FILTER CIRCUIT
(MAIN UNIT)
RF bandpass filters pass only the desired band signals and
suppress any undesired band signals.
The RF circuit has 7 RF bandpass filters (BPF) for signals
above 1.6 MHz and 1 low-pass filter (LPF) for signals and
suppress any undesired band signals.
(1) 0.03–1.6 MHz
The signals pass through the low-pass filter (L821, L822,
C822–C824) to suppress unwanted frequencies. The fil-
tered signals are bypassed a pre-amplifier by a BPF control
signal (B0) and preamp control signal (PROF), and are then
applied to the 1st mixer circuit (Q1101–Q1104).
(2) 1.6–2.0 MHz
The signals pass through a bandpass filter (L831–L833,
L835, L836, C831–C834, C837–C840) to suppress unwant-
ed frequencies. The filtered signals are then applied to the
pre-amplifier circuit.
• RECEIVER CONSTRUCTION
ANT
1st mixer
FI301A/B
Q1101–Q1104
Crystal
filter
64.455 MHz
1st LO

CIRCUIT DESCRIPTION

2nd mixer
FI1301
D401
Ceramic
BPF
455 kHz
2nd LO
(3) 2.0–30.0 MHz
The signals pass through a high pass filter (L811–L814,
C811–C817) to suppress excessively strong signals below
2.0 MHz, such as from broadcasting stations. The filtered
signals are applied to a low-pass filter or one of 5 bandpass
filters depending on their frequencies and are then applied
to the pre-amplifier circuit.

• Used RF filter

Control
Band
signal
0.03–1.6 MHz B0
1.6–2 MHz B1
2–4 MHz B2
4–8 MHz B3
4-1-3 PRE-AMPLIFIER CIRCUIT (MAIN UNIT)
The pre-amplifier circuit uses two 2SK2171s to obtain 10 dB
of gain over a wideband frequency range. When the pream-
plifier is turned ON, the signals above 1.6 MHz are applied
to the pre-amplifier circuit.
Q1401 and Q1402 are connected in parallel to easily match
the impedance to 50 Ω. IC3003 (pins 11, 12) switches the
signals from a bandpass filter, either to be bypassed, or to
be applied to the pre-amplifier, depending on the [PREAMP]
switch condition.
Amplified or bypassed signals are applied to the 1st mixer
circuit (Q1102–Q1104)
4-1-4 1ST MIXER CIRCUIT (MAIN 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.
SSB, CW, RTTY
Ceramic
BPF
FI611
AM
SSB-N,
CW-N,
RTTY-N
Crystal
BPF
(OPTION)
4 - 1
Input
Control
Band
diode signal
1 2
D802 8–11 MHz B4
D802 1 2 11–15 MHz B5
D803 15–22 MHz B6
D810 22–30 MHz B7
AM
detector
IC2001
to AF amplifier
AF
selector
BFO SQL controll
Input
diode
D804
D811
D805
D812