Receiver Circuits - Icom IC-R3 Service Manual

SECTION 4

4-1 RECEIVER CIRCUITS

4-1-1 RF SWITCHING CIRCUIT (RF UNIT)
(1) Below 1150 MHz
The RF signals from the antenna connector pass through
the band switching diode (D2) and an attenuator (D12, D13).
The signals are then applied to the RF circuit which sup-
press out-of-band signals via the band switching diode
(D911).
(2) Above 1150 MHz
The RF signals from the antenna connector pass through
the limitter (D1), band switching diode (D909) and high-pass
filter (C932–C936, C963, C964, L911, L912, L914). The fil-
tered signals are applied to the AF amplifier (IC14, pin 1),
and are then applied to the convertor circuit (IC13, pin 1).
Above 1150 MHz RF signals are mixed with "CONLO" signal
from CONV VCO circuit for convertion into below 1150 MHz
RF signals at the converter circuit (IC13). The converted sig-
nals are output from IC13 (pin 6), and are then applied to the
RF circuit which suppress out-of-band signals.
4-1-2 RF CIRCUIT (RF AND RF-B UNITS)
The RF circuit amplifies the received signals within the
range of frequency coverage and filters out-of-band signals.
(1) 0.495 MHz–29.995 MHz
RF signals (0.495 MHz–29.995 MHz) from the RF switching
circuit pass through a low-pass filter (C321–C325, L81,
L82). The filtered signals are amplified at an RF amplifier
(Q3) passing through each low-pass, bandpass, high-pass
filter depending on the receiving frequency. The amplified
signals are then applied to the 1st mixer circuit (IC1) via the
band switching diode (D10).
RF CIRCUIT
0.495 30 MHz
30 225 MHz
330 470 MHz
to the 2nd mixer circuit
D902
1st LO
470 800 MHz
IC1
FM-TV UNIT
225 330 MHz,
D1
800 1150 MHz
to the FM-TV circuit
CIRCUIT DESCRIPTION
BPF
Q3
RF
LPF
Q401, Q501,
Q601, Q701
RF
BPF BPF
BPF
Q801, Q851
RF
BPF BPF
The signals below 1.9 MHz pass through a low-pass filter
(C344–C347, L89, L90) between the band switching diode
(D4, D7), and are then applied to the RF amplifier circuit
(Q3).
The 1.9 MHz–14.995 MHz signals pass through the band
switching diode (D5) and bandpass filter (C332–C341,
L85–L88), and are then applied to the RF amplifier circuit
(Q3) via the band switching diode (D8).
The 15 MHz–29.995 MHz signals pass through the band
switching diode (D6) and high-pass filter (C326–C330, L83,
L84) and are then applied to the RF amplifier circuit (Q3) via
the band switching diode (D9).
(2) 30 MHz–224.995 MHz (RF-B UNIT)
RF signals (30 MHz–64.995 MHz, 65 MHz–107.995 MHz,
108 MHz–173.995 MHz, 174 MHz–224.995 MHz) from the
RF switching diode (RF unit; D11) are passed through the
low-pass filter (RF unit; C12–C17, L57–L59). The filtered
signals pass through the each bandpass filters and RF
amplifier depending on the receiving frequency. The signals
are then applied to the 1st mixer circuit (RF unit; IC1).
The 30 MHz–64.995 MHz signals pass through the band
switching diode (D403) and bandpass filter (D401), and are
then amplified at the RF amplifier (Q401). The amplified sig-
nals pass through the bandpass filter (D402) and band
switching diode (D404).
D4—D9 D3, D10
ANT ANT
LPF
SW SW
D403, D404
RF-B UNIT
D503, D504
D603, D604
D703, D704 D11
ANT ANT
LPF
SW SW
Q7 D21, D24
ANT
RF
BPF
SW
D803, D804
D853, D854
D805
ANT ANT
LPF
SW SW
D33, D35, D36
Q8, Q9
ANT
RF
BPF
SW
4 - 1
Above 1150 MHz RF signals
"CONLO" signal from
the CONV VCO circuit
D909, D912
ANT
SW
IC13
D2, D911
ANT
SW
Below 1150 MHz RF signals
CONVERTED RF FREQUENCY LIST
Displayed frequency RF frequency
1150.000—1176.000 141.000—115.000
1176.005—1229.995 143.995—90.005
1230.000—1255.000 61.000—36.000
1255.005—1286.000 64.995—34.000
1286.005—1320.995 64.995—30.005
1321.000—1349.995 30.000—58.995
1350.000—1384.995 30.000—64.995
1385.000—1394.995 94.000—103.995
1395.000—1427.995 75.000—107.995
1428.000—1515.995 137.000—224.995
1516.000—1569.995 196.000—249.995
1570.000—1579.995 250.000—259.995
1580.000—1739.995 289.000—448.995
1740.000—1789.995 420.000—469.995
1790.000—1809.995 499.000—518.995
1810.000—1849.995 490.000—529.995
1850.000—2039.995 559.000—748.995
2040.000—2109.995 720.000—789.995
2110.000—2204.995 819.000—913.995
2205.000—2250.995 885.000—930.995
2251.000—2450.095 960.000—1159.095
ANT