Section 4 Circuit Description; Receiver Circuits - Icom ic-r20 Service Manual

SECTION 4

4-1 RECEIVER CIRCUITS

4-1-1 BAND SWITCHING CIRCUIT (RF UNIT)
The RF signals from the antenna connector pass through
the limiter (D68) and an attenuator* (D69). The signals are
then applied to the antenna switching circuit (D3, D11, D13,
D65, D66 and D73–D75 ).
*Above 2 GHz RF signals do not pass through the attenua-
tor.
4-1-2 RF CIRCUIT (RF UNIT)
The RF circuit amplifies the received signals within the
range of frequency coverage and filters out-of-band signals.
• RF CIRCUITS
from the
bar antenna
MF (0.15 1.9 MHz)
ANT
SW
BAND
RF
SW
Q505
30 300M (30 300 MHz except A-VHF range)
Q36
RF
BAND
SW
BAND
SW
IC18
RF
IC19
RF
CIRCUIT DESCRIPTION
LPF
HF-L (1.9 15 MHz)
BAND
BPF
SW
MF (15 30 MHz)
HPF
BAND
BPF
SW
RF-B circuit
1st mixer
to 2nd mixer
IC15
circuit
RF-A circuit
1st mixer
to 2nd mixer
circuit
IC1
• A-BAND CIRCUIT
(1) MF (above 0.15 MHz, below 1.9 MHz) signals
RF signals (0.15–1.9 MHz) from an attenuator (D69) pass
through the low-pass filter (L123–L125 and C851–C853),
band switch (D66) and another low-pass filter (L88, L89,
C533–C535, C657, C658). The filtered signals pass through
another band switch (D67), and are then amplified at an RF
amplifier (Q505). The amplified signals are applied to the
next band switch (D72).
(2) HF-L (above 1.9 MHz, below 15 MHz) signals
RF signals (1.9–15 MHz) from an attenuator (D69) pass
through the low-pass filter (L123–L125 and C851–C853),
band switch (D65) and bandpass filter (L85–L87, L91,
C522–C531 and C891). The filtered signals pass through
another band switch (D70), and are then amplified at an RF
amplifier (Q505). The amplified signals are applied to the
next band switch (D72).
LO (1001 or 2002MHz)
IC3 IC2
RF
mixer
LPF
Q550
Pre
Amp
A-UHF (300 470 MHz)
BPF RF
Q35
B-UHF (300 470 MHz)
BPF RF
Q548
800M (470 833 MHz)
BPF RF BPF
Q24
1G (833 1305 MHz)
BPF
RF
Q25
4 - 1
ANTENNA
1305 3305 MHz
BAND
HPF
SW
BPF
A-VHF (118 175 MHz)
BPF
RF
Q14
B-VHF (118 175 MHz)
BPF
RF
Q549
Q551
Pre
LPF
Amp
BAND
SW
LIMITER
ATT
HPF