Receiver Circuits - Icom IC-F50V Service Manual

SECTION 4

4-1 RECEIVER CIRCUITS

RF CIRCUITS
RF circuits consist of RF fi lters, antenna switch (ANT SW),
RF amplifi er (RF AMP), etc., and extracts and amplifi es the
signals of frequency which desired to receive.
The received signals (RX signals) from the antenna are passed
through the LPF, ANT SW (as an LPF in RX), limitter , BEF
(Band Eliminate Filter) and the two-staged tuned BPF. The
fi ltered RX signals are amplifi ed by the RF AMP, and passed
through another two-staged tuned BPF. The fi ltered RX signals
are then applied to the 1st IF circuits.
The ANT SW toggles RX line and TX line. While receiving,
the TX line and the antenna is disconnected to prevent RX
signals entering. The RX line is disconnected from the GND
simultaneously, and an LPF which guides received signals
to the RX circuits is composed.
• RF CIRCUITS
LPF
L1 2 L14
C49
1stLO
C4 1 C40
Two-staged
TUNED BPF
1ST IF CIRCUITS
The 1st IF circuits consist of 1st mixer, 1st IF fi lter and 1st IF
amplifi er (IF AMP). And it converts the RX signals into the
1st IF signal, then filters to remove unwanted signals and
amplifi es.
The filtered RX signals are applied to the 1st mixer to be
converted into the 1st IF signal, by being mixed with the 1st
Local Oscillator (LO) signals "1stLO" from the RX VCO via
the LPF.
• 1ST IF CIRCUITS
R5V
Q4
CIRCUIT DESCRIPTION
R5V
R172
C186
RF
AMP
C37
Q2
C29
R2 2
1st IF
FILETR
1st IF
FI1
AMP
4 1
C5 8 R3 3 GND IN
2 3
OU T GND
FL-335
C329
LIMITTER
While transmitting, serial-connected PIN diodes are ON,
thus the TX line is connected to the antenna, and the RX
line is connected to the GND simultaneously to prevent TX
signal entering.
The limitter protects RX line from over-level RF inputs, and
the BEF (=trap) damps unwanted signals to GND.
The tuned-BPF is adjusted so that it responds to receiving
frequency and rejects all others, by the variable capacitor
whose capacitance is varied by added voltage "T1" and "T2."
The RF AMP amplifi es RX signals to a level suited to the 1st
mixer.
AGC LINE
Q1
DRIVER
R174
Two-staged
TUNED BPF
C2 2 C25
6P
D4
R14 BEF
The converted 1st IF signal is passed through the 1st IF fi lter
to be removed unwanted signals. The fi ltered 1st IF signal is
applied to the 1st IF AMP via the limitter. The amplifi ed 1st
IF signal is then applied to the 2nd IF circuits.
R5V
R2 7
L1
1stLO
R32
C5 1
1
C4 1
Q3
1st
MIXER
Two-staged
TUNED BPF
4 - 1
To TX AMP
LIMITTER
C6
C19
L6 L5
LPF (ANT SW)
R5V
LPF
2 L14
C49
C40
R2 2
To ANT
L1
L2
C2
C4
LPF