Circuit Description; Receive Circuits - Icom ID-880H Service Manual

SECTION 4.

4-1 RECEIVE CIRCUITS

RF CIRCUITS
144 MHz BAND (118–174 MHz)
The RX signals (118–174 MHz) from the antenna are
passed through two LPFs and TX/RX SW for VHF band (D59,
68, 75, 77), and applied to the RF circuit.
The RX signals are passed through the limiter (D78, 79) and
BPF, and amplifi ed by the RF AMP (Q33). The amplifi ed RX
signals are filtered by tuned BPF (D41, 44, 60) to remove
unwanted signals, then applied to the 1st mixer (Q124).
The passband frequency of the tuned BPF (D41, 44, 60) is
tuned by the tuning voltage from the D/A converter controlled
by the CPU (IC25).
300 MHz BAND (230–420 MHz)
The RX signals (230–420 MHz) from the antenna are
passed through the LPF, HPF and TX/RX SW for UHF band
(D30, 53, 61, 62, 177), and applied to the RF circuit.
The RX signals are passed through the RX SW (D56) and
filtered by the tuned BPF (D57, 58) to remove unwanted
signals, then applied to the RF AMP (Q35).
The amplifi ed RX signals are fi ltered by another tuned BPF
(D45, 51) to remove unwanted signals, and amplified by
another RF AMP (Q34). The RF AMP (Q34) improves the
isolation between the 1st mixer (IC64) and the tuned BPF.
The amplifi ed RX signals are applied to the 1st mixer (IC64).
The pass-band frequencies of the tuned BPFs (D57, 58:
D45, 51) are tuned by the tuning voltage from the D/A
converter controlled by the CPU (IC25).
440 MHz BAND (420–550 MHz)
The RX signals (420–550 MHz) from the antenna are
passed through the LPF, HPF and TX/RX SW for UHF band
(D30, 53, 61, 62, 177), and applied to the RF circuit.
The RX signals are passed through the RX SW (D16) and
tuned BPF (D13) to remove unwanted signals, and applied
to the RF AMP (Q19). The amplifi ed RX signals are fi ltered
by another tuned BPF (D12, 14, 23) to remove unwanted
signals, then applied to the 1st mixer (Q125).
The pass-band frequencies of tuned BPFs (D13: D12,
14, 23) are controlled by the tuning voltage from the D/A
converter controlled by the CPU (IC25).

CIRCUIT DESCRIPTION

800 M Hz BAND (810–999 MHz)
The RX signals (810–999 MHz) from the antenna are
passed through the HPF, and applied to the RF AMP (Q28).
The amplifi ed RX signals are fi ltered by the tuned BPF (D18,
20) to remove unwanted signals, then applied to the 1st
mixer (IC64) via the RX SW (D9).
The pass-band frequency of tuned BPF (D18, 20) is tuned
by the tuning voltage from the D/A converter controlled by
the CPU (IC25).
• RF CIRCUITS
4 - 1
D59,68,75,77
TX/RX
From VHF TX circuit
SW
D30,53,61,62,177
From UHF TX circuit
D41,D44,D60
To the 1st IF mixer (Q124)
BPF
BPF1,2,3
D12,14,23
To the 1st IF mixer (Q125)
BPF
BPF1,2,3
<- To the 1st IF mixer (IC64)
D45,D51
D36
Q34
RX RF
BPF
SW AM P
BPF2,3
<- To the 1st IF mixer (IC64)
D9
D18,D20
RX
BPF
SW
BPF2,3
LPF LPF
TX/RX
HPF
SW
UHFLPF
VHFLPF
D78,D79
Q33
RF
LIMITER
BPF
AM P
<- 118-174 MHz
D16
D 13
Q19
RF RX
BPF
AM P SW
<- 420-550 MHz
BPF1
D57,58
D56
Q35
RF RX
BPF
AM P SW
<- 230-420 MHz
BPF1
Q28
<- 810-999 MHz
RF
HPF
AM P