SECTION 3 CIRCUIT DESCRIPTION
3 - 1
3-1 RECEIVER CIRCUITS
3-1-1 RF SWITCHING CIRCUIT (MAIN UNIT)
The IC-R75 has two antenna connectors. RF signals enter
either the [50
Ω
ANT.] or [450
Ω
ANT.] connector.
RF signals from the [50
Ω
ANT.] connector are applied to the
antenna switching circuit (RL121), and then pass through the
low-pass filter (L131, L132, C131–C136).
RF signals from the [450
Ω
ANT.] connector are passed
through the L101 to exchange the impeadance value, and
are then applied to the antenna switching circuit (RL121).
The signals are applied to the low-pass filter (L131, L132,
C131–C136).
Each RF signals from the [50
Ω
ANT.] connector or [450
Ω
ANT.] connector are chosen by the antenna switching circuit
(RL121).
3-1-2 RF FILTER CIRCUIT (MAIN UNIT)
The filtered signals are applied to the RX attenuator switch-
ing circuit (RL141). Either the signals bypass or pass through
the attenuator circuit. The signals are attenuated at 20 dB
when passing through the attenuators. The attenuator sys-
tem excludes non-linear components between an antenna
connector and an attenuator to prevent strong signals from
causing distortion. The signals are then applied to the RF fil-
ters. The MAIN UNIT has 8 RF bandpass fileters for signals
above 2.0 MHz and 2 low-pass filters for signals below 2.0
MHz.
(1) Below 1.6 MHz
The signals are applied to the low-pass filter consisting of
C170–C175, L171–L173 via the limitter circuit (D141, D142).
A diode is removed at the entrance of the low-pass filter. This
device prevents the diode from causing distortion when
receiving very strong signals. A switching diode (D172) is
turned on when the “B0” line is “HIGH”.
(2) Above 1.6 MHz
The signals are applied to the high-pass filter consisting of
C161–C163, L161–L164. This filter suppresses strong sig-
nals below 1.6 MHz such as broadcasting stations.
The filtered signal between 1.6 MHz and 2.0 MHz are applied
to the low-pass filter (C182–C187, L182, L183) via the
switching diode (D181). The switching diodes (D181, D182)
are turned ON when the “B1” line is “HIGH”.
The filtered signals above 2.0 MHz are applied to one of 8
bandpass filters depending on the receive frequencies.
After passing through a bandpass or low-pass filter, the sig-
nals are applied to the pre-amplifier circuit (Q381, Q382,
IC391).
(3) FILTER SWITCHING CIRCUIT
The RF bandpass filter corresponds to the BPF switching
voltage (B0–B9) based on the CPU via the shifit registor
(IC551, IC552) and driver (IC561, IC562). The switching volt-
age of the BPF exit ot improve multi-signal and strong signal
characteristics.
• RF bandpass and preamplifier circuit
ANT1 (50
Ω
)
ANT2 (450
Ω
)
ATT (20 dB)
HPF
LPF
0.5 – 1.6 MHz
1.6 – 54 MHz
ATT (10 dB)
LPF
B0
1.6 – 2.0 MHz
B1
BPF
2.0 – 4.0 MHz
B2
BPF
50 – 60 MHz
B9
PRE
AMP
Q381, Q382
PRE
AMP
IC391
LPF
60 MHz
to 1st mixer
(Q441, Q442)
preamplifier circuit
