Receiver Circuits - Icom IC-2720 Service Manual

SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 TRIPLEXER AND RX BAND SWITCHING
CIRCUITS (MAIN UNIT)
The transceiver has a triplexer (low-pass and high-pass fil-
ters) on the first stage from the antenna connector to sepa-
rate the signals into VHF and UHF signals. The RF signals
from the antenna connector are applied to the tripler or RX
band swtich circuits.
• RF SIGNALS V-V (118 MHz–180 MHz), U-V (136
MHz–174 MHz)
The V-V and U-V RF signals from the antenna connector
pass through the low-pass filter (L76, L77, L80, C205, C209,
C242, C243, C264), and then applied to the TX/RX switching
circuit (D42, D46, D52, D67, D2049). The filtered signals are
amplified at the pre-amplifier (Q33), and are applied to the
left side or right side displayed RX circuits.
• RF SIGNALS U-U, V-U2 (375 MHz–550 MHz)
The U-U and V-U2 RF signals from the antenna connector
pass through the high-pass filter (L78, L81, C206, C210,
C213, C265), and then applied to the TX/RX switching circuit
(D55, D64, D65, D2050) via the SWR detector (D50, D58).
The filtered signals are amplified at the pre-amplifier (Q24),
and are applied to the left side or right side displayed RX cir-
cuits.
• RF SIGNALS U-U3 (810 MHz–1000 MHz)
The U-U3 RF signals from the antenna connector pass
through the two low-pass filters (L76, L77, L80, C205, C209,
C242, C243, C264, L87, L88, C266–C268), and are then
applied to the RX band swtiching circuit (D2061). The filtered
signals are amplified at the RF amplifier (Q18), and are
applied to the right side displayed RX circuits.
• RF CIRCUIT FOR LEFT SIDE DISPLAY
1st mixer
(IC1005)
to 2nd mixer
circuit
1st LO
RX
BPF
SW
D28 D32, D39 Q29
RX
ATT BPF
SW
D30 D34
RX
ATT BPF
SW
D31 D35
RX
BPF
SW
D9 D13, D17 Q20
• RF SIGNALS V220 (174 MHz–260 MHz), V-U1 (225
MHz–375 MHz)
The V220 and V-U1 RF signals from the antenna connector
are applied to the RX band swtiching circuit (Q34, D66, RL1),
and are applied to the left side displayed RX circuit.
4-1-2 RF CIRCUIT FOR LEFT SIDE DISPLAY
(MAIN UNIT)
• RF SIGNALS V-V (118 MHz–180 MHz)
The amplified signals are applied to the RF amplifier (Q29)
after being passed through the attenuator (D59) and band-
pass filter (D47, D53). The signals are applied to the RX
band switching circuit (D28) via the another bandpass filter
(D32, D39) to supress the unwanted signals.
• RF SIGNALS V220 (174 MHz–260 MHz)
The signals are applied to the RF amplifier (Q31) after being
passed through the RX band switching circuit (D62) and
bandpass filter (D51). The amplified signals are applied to
the RX band switching circuit (D28) via the another bandpass
filter (D34) to supress the unwanted signals and attenuator
(R195–R197).
• RF SIGNALS V-U1 (225 MHz–375 MHz)
The signals are applied to the RF amplifier (Q32) after being
passed through the RX band switching circuit (D63) and
bandpass filter (D49). The amplified signals are applied to
the RX band switching circuit (D31) via the attenuator
(R198–R200) and another bandpass filter (D35) to supress
the unwanted signals.
• RF SIGNALS V-U2 (375 MHz–550 MHz)
The amplified signals are applied to the RF amplifier (Q20)
after being passed through the attenuator (D25) and band-
pass filter (D23, D73). The signals are applied to the RX
band switching circuit (D9) via the another bandpass filter
(D13, D17) to supress the unwanted signals.
V-V (118 MHz—174 MHz)
RF
BPF ATT
D47, D53 D59
V220 (174 MHz—260 MHz)
RF
BPF
Q31 D51
V-U1 (225 MHz—375 MHz)
RF
BPF
Q32 D49
V-U2 (375 MHz—550 MHz)
RF
BPF
ATT
D23, D73 D25
4 - 1
ANTENNA
PRE
Q33
RX
SW
D62
RX
SW
Q34, D66,
RL1
RX
SW
D63
PRE
Q24