Circuit Description; Receiver Circuits; Antenna Switching Circuit; Rx-A Circuit - Icom IC-7800 Service Manual

SECTION 3 CIRCUIT DESCRIPTION

3-1 RECEIVER CIRCUITS

3-1-1 ANTENNA SWITCHING CIRCUIT

(ANT-SW UNIT)
The ANTENNA switching circuit leads receive signals to
bandpass filters 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 are applied to the RX-A, RX-B or RX-ONLY
circuits.

• RX-A CIRCUIT

RF signals from [ANT1], [ANT2], [ANT3] or [ANT4] pass
through one of the antenna selectors (RL51; passing
through [ANT1], RL61; passing through [ANT2], RL71;
passing through [ANT3], RL81; passing through [ANT4]),
and are then applied to the CTRL board via J1.
The RF signals pass through the current detector (CTRL
board; D301, D302), and are then applied to the BPF unit
via J351 on the CTRL board.
• RX-B CIRCUIT
RF signals from [ANT1], [ANT2], [ANT3] or [ANT4] pass
through one of the antenna selectors (RL52; passing
through [ANT1], RL62; passing through [ANT2], RL72;
passing through [ANT3], RL82; passing through the ANT4),
and are then applied to the BPF unit via J2.
• RX-ONLY CIRCUIT
RF signals from [ANT4] pass through the antenna selector
(RL84), and are then applied to the BPF unit via J3.
3-1-2 BANDPASS FILTER CIRCUIT
(BPF, AND DIGISEL UNITS)
RF bandpass filters pass only the desired band signals and
suppress any undesired band signals. The RF circuit has
11 bandpass filters and 1 low-pass filter.
• RX-A OR RX-ONLY CIRCUITS
RF signals from RX-A circuit (CTRL board; J351) or
RX-ONLY circuit (ANT-SW unit; J3) are applied to the RF
signal switching circuit (BPF unit; RL402). The filtered
signals pass through (or bypass) splitter (BPF unit; L371–
L373) via the RL371 and RL355 (BPF unit), and are then
attenuated at RL354 (3 dB), RL353 (6 dB) and RL352 (12
dB) on the BPF unit. A portion of the filtered signals are
separated at the splitter, and are then applied to the RX-B
circuit's DW selector (BPF unit; RL855).
The attenuated signals pass through the low-pass filter or
one of the bandpass filters via the 6M selector (BPF unit;
RL851).
(1) 0.03–30 MHz (BPF AND DIGISEL UNITS)
The signals pass through the low-pass filter or one of the
9 bandpass filters. The filtered signals pass through (or
bypass) the bandpass filter and the RF amplifier (DIGISEL
unit; Q201) via the D_S_THRU selectors (RL21, RL2) on
the DIGISEL unit, and are then applied to the pre-amplifier
circuit on the RXPLL-A unit.
(2) 50–60 MHz (BPF UNIT)
The signals pass through the one of the 2 bandpass filters,
and are then amplified (or bypass) at the DW amplifier
(Q901) via the DW selectors (RL900, RL901). The signals
are applied to (or bypass) the pre-amplifier circuit.
• RX-B CIRCUIT
RF signals from RX-B circuit (ANT-SW unit; J2) pass
through the REV_Po detector (BPF unit; Q873, D872). The
signals are applied to the DW selector (BPF unit; RL855).
A portion of RF signals from RX-A or RX-ONLY circuits are
applied to the this selector too.
The signals are attenuated at RL854 (3 dB), RL853 (6 dB)
and RL852 (12 dB) on the BPF unit, and are then applied
to the 6M selector (BPF unit; RL851).
The attenuated signals pass through the low-pass filter or
one of the bandpass filters.
(1) 0.03–30 MHz (BPF AND DIGISEL UNITS)
The signals pass through the low-pass filter or one of the
9 bandpass filters. The filtered signals pass through (or
bypass) the bandpass filter and the RF amplifier (DIGISEL
unit; Q701) via the D_S_THRU selectors (RL521, RL502)
on the DIGISEL unit, and are then applied to the pre-
amplifier circuit on the RXPLL-B unit.
(2) 50–60 MHz (BPF UNIT)
The signals pass through the one of the 2 bandpass filter,
and are then amplified (or bypass) at the DW amplifier
(Q951) via the DW selectors (RL950, RL951). The signals
are applied to the pre-amplifier circuit.
3-1-3 PRE-AMPLIFIER CIRCUITS
( BPF, RXPLL-A, AND RXPLL-B UNITS)
The IC-7800 has 2 gain levels of pre-amplifier circuits
which is composed of the pre-amplifier1 and pre-amplifier2.
The circuit is controlled by [P.AMP] switch.
• RX-A OR RX-ONLY CIRCUITS
(1) 0.03–30 MHz (RXPLL-A UNIT)
The RF signals from the DIGISEL unit are applied to (or
bypass) the DW amplifier (Q1001) via the DW selectors
(RL1001, RL1002), and are then applied to the pre-
amplifier circuit.
When [P.AMP] switch is set to [P.AMP 1] or [P.AMP 2], the
signals are applied to the pre-amplifier1 (Q1007, Q1008) or
pre-amplifier2 (Q1006) circuit via the P.AMP_ON selectors
(RL1003, RL1006) and P.AMP selectors (RL1004, RL1005),
respectively. Pre-amplified or bypassed signals are applied
to the 1st mixer circuit after being passed through the low-
pass filter (L1052–L1054, C1053–C1057, C1067).
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