Circuit Description; Receiver Circuits - Icom ID-800H Service Manual

SECTION 4

4-1 RECEIVER CIRCUITS

4-1-1 TRIPLEXER CIRCUIT (MAIN UNIT)
The transceiver has a triplexer (low-pass and high-pass
filters) on the first stage from the antenna connector
(CHASSIS; J2) to separate the signals into VHF, UHF and
800 MHz band signals. The 2 of low-pass filters (L51, L52,
L56, C295, C299 and L45, L46, L49, C282, C285, C289)
are for VHF band signals, the low-pass (L51, L52, L56,
C295, C299) and high-pass (L47, L50, C284, C288, C292,
C586) filters are for UHF band signals and high-pass filter
(L55, L58, C298, C302, C304) is for 800 MHz band signals.
The separated signals are applied to each antenna switching
circuits .
4-1-2 ANTENNA SWITCHING CIRCUITS (MAIN UNIT)
The antenna switching circuit (VHF: D50, UHF:
functions as a low-pass filter while receiving. However,
its impedance becomes very high while transmitting.
Thus transmit signals are blocked from entering the
receiver circuits. The antenna switching circuit employs a
λ type diode switching system.
1
⁄
4
800 MHz band signals pass through the antenna switch
(D44).
The passed signals are then applied to the each attenuator
circuits.
4-1-3 ATTENUATOR CIRCUITS (MAIN UNIT)
The attenuator circuit attenuates the signal strength to a
maximum of 10 dB to protect the RF amplifier from distor-
tion when excessively strong signals are received.
• RF AND IF CIRCUITS
to the 2nd IF circuit

CIRCUIT DESCRIPTION

D8
IF
BPF
SW
1st mixer (Q32)
D9
IF
BPF
SW
1st mixer (Q35)
D17
D10
IF BAND
SW SW
1st mixer (IC12)
Q37
RF
BAND
AMP
SW
D18
The RF signals from the antenna switching circuits pass
through the each attenuator circuit (for VHF band is D49, for
UHF band is D46 and D48, for 800 MHz band is D39).
The D/A converter outputs "ATT" signal (IC11, pin 6), and is
then applied to the attenuator controller (Q44). The circuit
output attenuator control signals to each attenuator circuits.
The attenuated signals are applied to the each RF circuits.
4-1-4 RF CIRCUITS (MAIN UNIT)
The RF circuit amplifies signals within the range of frequen-
cy coverage and filters out-of-band signals.
• VHF BAND RF CIRCUIT (118–174 MHz)
The RF signals from the attenuator circuit pass through the
tunable bandpass filter (D41, L40, L44, C273, C280). The
D51)
filtered signals are amplified at the RF amplifier (Q38) and
are then applied to the another 3-stage tunable bandpass
filter (D22, D23, D29, L28, C171, C174, C176, C177, C181,
C185, C191, C198, C201, C204, C208) to suppress unwant-
ed signals and improve the selectivity. The filtered signals
are applied to the VHF 1st mixer circuit (Q32).
• UHF BAND RF CIRCUIT
(EXP: 430–440 MHz, USA: 440–450 MHz)
The RF signals from the attenuator circuit are applied to the
RX switch (D42). The switched signals are amplified at the
RF amplifier (Q39) and are then applied to the bandpass
filter (FI4) to suppress unwanted signals. The filtered signals
are applied to the UHF 1st mixer circuit (Q35).
Q38
RF
BPF ATT
AMP
UHF band (EXP: 430−440 MHz, USA: 440–450 MHz)
Q39
RF
BAND
ATT
AMP
SW
D42
Q36 Q40
RF
BPF
AMP
Q41
RF
BPF
BPF
AMP
4 - 1
VHF band (118−174 MHz)
T/R
LPF
SW
T/R
HPF
SW
400M band (230−550 MHz)
RF
BAND
BPF
AMP
SW
D43
800M band (810−1000 MHz)
ANT
ATT
SW
D44
Antenna
LPF
HPF