Receiver Circuits - Icom IC-T70A Service Manual

SECTION 4.

4-1 RECEIVER CIRCUITS

DUPLEXER
The VHF/UHF RX signal from the antenna is separated by
frequency in the duplexer. The VHF signal is passed through
the LPF, which eliminates the unwanted VHF and above
frequencies, and the UHF signal is passed through the HPF,
which eliminates the frequencies below UHF.
RF CIRCUITS
• VHF BAND
The VHF RX signal from the duplexer is passed through
the LPF, ANT SW (D509–D511) and BPF, and then applied
to the low-noise RF AMP (Q505). The amplified signal
is filtered by the 3-stage tuned BPF (D502–D504) to
thoroughly eliminate unwanted frequencies. (e.g. the image
frequencies, etc.). The filtered RX signal is then applied to
the 1st IF circuits.
• UHF BAND
The UHF RX signal from the duplexer is passed through two
LPFs, ANT SW (D506, D524) and tuned BPF (D523), and
then applied to the low-noise RF AMP (Q511). The amplified
signal is filtered by the tuned BPF (D512, D515, D519) to
thoroughly eliminate unwanted frequencies. The filtered RX
signal is then applied to the 1st IF circuits.
• DUPLEXER AND RF CIRCUITS
D502,D503,D504
To the 1st IF circuits
BPF
D512,D515,D519
To the 1st IF circuits
BPF
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CIRCUIT DESCRIPTION
D506, D524
ANT
UHF TX circuits
SW
D510, D511
D509,
ANT
VHF TX circuits
SW
Q505
RF
BPF
AMP
D523
Q511
RF
BPF
AMP
1ST IF CIRCUITS
The signal from the RF circuits is applied to the 1st IF
mixer (VHF band; Q500/UHF band; Q509) and mixed with
the 1st LO signal from the VHF/UHF VCO, resulting in the
46.35 MHz 1st IF signal. The 1st IF signal is filtered by the
crystal filter (FI301), amplified by the 1st IF AMP (Q318),
and then applied to the 2nd IF circuits through the limiter
(D313).
• 1ST IF CIRCUITS
D313
Q318
To the
2nd IF circuits
IF
LIMIT
AMP
400-479 MHz
LPF LPF
136-174 MHz
LPF
4 - 1
VHF
LO signal
VHF 1st mixer
From the VHF
Q500
RF circuits
UHF
LO signal
UHF 1st mixer
From the UHF
Q509
RF circuits
FI301
46.35MHz
XTAL
BPF
Duplexer
HPF
LPF