Receiver Circuits - Icom IC-91A Service Manual

SECTION 5

5-1 RECEIVER CIRCUITS

This transceiver has two receiving lines called A_BAND and B_
BAND for dualwatch capability. A_BAND corresponds to FM/WFM/
AM mode receiving within 0.495–999 MHz range, and B_BAND
corresponds to FM/FM-N/AM/DV mode receiving within 118–174
MHz and 350–470 MHz ranges.
5-1-1 RF CIRCUITS (RF UNIT)
This transceiver has six RF circuits to provide wide receiving range.
The received signals from the antenna connector (CHASSIS; J1)
are applied to each RF circuit for the frequency coverage, and
amplified within the frequency coverage.
• While receiving 0.495–76 MHz
The received signals of 76 MHz and below are passed through the
low-pass filter (LPF; L200, L201, C201–C204) via the two LPFs
(L1–L3, C1–C6; L5–L7, C10–C14), attenuator (D10) and band
switch (D200).
The 0.495–30 MHz band signals are passed through the band
switch (D201), then applied to the RF amplifier (Q200) via the
low-pass filter (L202, L203, C205–C209). The 30–76 MHz band
signals are applied to the tuned RF amplifier (Q250, D252, D255)
via the band switch (D250).
The amplified signals are applied to the A_BAND 1st mixer (IC900,
pin 1) via the band switches (D203 or D256, D707).
- 0.495–76 MHz -
Q200 0.495-30 MHz
RF
amp.
to the A_BAND
D203
1st mixer
(IC900, pin 1)
D707
Tuned RF amplifier
D256
D255 Q250
30-76 MHz
• While receiving 76–118 MHz or 174–260 MHz
The 76–118 MHz band signals are passed through the two LPFs
(L1–L3, C1–C6; L5–L7, C10–C14), attenuator (D10) and low-pass
filter (L300, L301, C301–C305) via the band switch (D300).
The 174–260 MHz band signals are passed through the LPF (L1–
L3, C1–C6), HPF (L4, C7–C9), attenuator (D7) and high-pass filter
(HPF; L302, L303, C306–C310) via the band switch (D302).
The filtered signals are applied to the tuned RF amplifier (Q300,
Q301, D305, D307, D308) via the band switch (D301/D303).
The amplified signals are applied to the A_BAND 1st mixer (IC900,
pin 1) via the band switches (D311, D707).
CIRCUIT DESCRIPTION
2 LPFs
LPF
LPF
D201
0.495-76 MHz
D10
LPF
ATT
D200
D250
D252
- 76–118 MHz or 174–260 MHz -
D303
174-260 MHz
D301
76-118 MHz
D707 D311
to the A_BAND
1st mixer
(IC900, pin 1)
• While receiving 118–174 MHz
The 118–174 MHz band signals are passed through the two LPFs
(L1–L3, C1–C6; L5–L7, C10–C14) and attenuator (D10), then
applied to the RF amplifier (Q400) via the band switch (D400) and
bandpass filter (BPF; L400, L405, C400, C401).
The amplified signals are applied to the tuned RF amplifiers for A_
BAND (Q401, D401–D403) and B_BAND (Q450, D450–D452).
The amplified signals by Q401 are then applied to the A_BAND
1st mixer (IC900, pin 1) via the band switches (D405, D509).
The amplified signals by Q450 are then applied to the B_BAND
1st mixer (MAIN UNIT; IC50, pin 1) via the band switch (D454) and
J1001 (pin 15).
- 118–174 MHz -
from the
antenna
to the A_BAND
1st mixer
D402, D403 Q401
(IC900, pin 1)
D509 D405
D451, D452 Q450
D454
to the B_BAND
1st mixer
(MAIN UNIT; IC50, pin 1)
• While receiving 260–350 MHz or 470–600 MHz
The 260–350 MHz and 470–600 MHz band signals passed through
the LPF (L1–L3, C1–C6) and HPF (L4, C7–C9), then applied to
the tuned RF amplifier (Q500, Q501, D502, D505, D506) via the
attenuator (D7) and band switch (D500).
The amplified signals are applied to the A_BAND 1st mixer (IC900,
pin 1) via the band switches (D508, D509).
- 260–350 MHz or 470–600 MHz -
D509 D508
to the A_BAND
1st mixer
(IC900, pin 1)
5 - 1
D7
D302
HPF HPF
ATT
D10
D300
LPF LPF
ATT
Q301 D307, D308 Q300 D305
Tuned RF amplifier
Tuned RF amplifier
D401 D10
ATT
Tuned RF amplifier
Q400
D450
RF
amp.
D7
D500
ATT HPF LPF
260-350 MHz, 470-600 MHz
Q501 D505, D506 Q500 D502
Tuned RF amplifier
from the
antenna
LPF
from the
antenna
2 LPFs
LPF
118-174 MHz
BPF
D400
from the
antenna