Icom IC-A20MK II Service Manual page 6

SECTION 3
CIRCUIT DESCRIPTION
3-1 RECEIVER CIRCUITS
3-1-1 ANTENNA SWITCHING CIRCUIT
(RF UNIT)
The antenna switching circuit functions as a low-pass filter
while transmitting and as a resonator circuit while receiving.
Receive signals enter the RF UNIT from the antenna con-
nector (02001) and pass through a Chebyschev low-pass
filter (C2001-C2005, L2001, L2002). The antenna switch-
ing circuit employs а A /4-type diode switching system.
While receiving, D2001 and D2002 turn OFF, and the
received signals which are applied to the RF circuit pass
through a low-pass filter (C2007 -C2010, 12003, 12004).
3-1-2 RF CIRCUIT (RF UNIT)
The signals pass through a bandpass filter (C2012~C2015,
D2003, D2004, L2005, L2006) and are then amplified at an
RF amplifier (Q2002). The received signals are applied to a
bandpass filter (C2021-C2023, D2005, D2006, L2007,
1.2008).
D2003-D2006 are varactor diodes and are controlled by the
PLL lock voltage. The voltage is current-amplified at Q2023
andis then applied to the varactor diodes. These varactor
diodes tune the center frequency of an RF passband for
wide bandwidth receiving and good image response rejec-
tion.
3-1-3 1st MIXER AND 1st IF CIRCUITS
(RF UNIT)
The signals from the bandpass filter are mixed with a 1st LO
signal from the VCO UNIT and are converted to a 35.8 MHz
ist IF signal. The 1st IF signal is applied to a pair of crystal
filters (FI2001) to suppress out-of-band signals and is then
amplified at the 1st IF amplifier (Q2004).
RECEIVER CIRCUIT BLOCK DIAGRAM
MAIN ; RF
UNIT ; UNIT
CRYSTAL
FILTER
Fi2001
to
IC1004
from AGC from VCO
Fig. 3-1
3—1
from BPF control
3-1-4 2nd LO AND 2nd MIXER CIRCUITS
(MAIN UNIT)
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal.
The 1st IF signal from the 1st IF circuit is applied to the 2nd
mixer section of 1C1004 (pin 16) and is mixed with a 2nd LO
signal for conversion to a 455 kHz 2nd IF signal.
IC1004 contains the 2nd LO, 2nd mixer, limiter, quadrature
detector and noise amplifier circuits. The 2nd LO circuit
section in 1C1004 and X1001 generate a 35.345 MHz 2nd
LO signal.
To suppress unwanted heterodyned frequency signals, the
2nd IF signal from the 2nd mixer section of ІС1004 (pin 3) is
applied to the ceramic filter (FL1001 or FL1002).
FL1001 is for the communication band; FL1002 is for the
navigation band and allows a VOR signal that is modulated
by a 9960 Hz signal to pass through to the 2nd IF amplifiers
(Q1001, Q1002).
Depending on whether the communication band or navi-
gation band is in use, the CPU (1C3001) selects either filter
(FL1001 or FL1002) using the filter selector circuit (D1001,
D1002, Q1004).
On the navigation band, Q1006 turns OFF and Q1004 turns
ON, and the 2nd IF signal passes through FL1002. On the
communication band, Q1006 turns ON and Q1004 turns
OFF, and the 2nd IF signal passes though FL1001.
3-1-5 AM DETECTOR CIRCUIT (RF UNIT)
The AM detector circuit demodulates an AM signal on the
communication band and navigation band.
The amplified signal from Q1002 is detected at the AM
detector (D1003) for conversion to an AF signal.
from AGC