Circuit Description; Receiver Circuits - Icom IC-PCR100 Service Manual

SECTION 4

4-1 RECEIVER CIRCUITS

4-1-1 RF ATTENUATOR CIRCUIT
The attenuator circuit attenuates the signal strength to
approx. 20 dB to protect the RF amplifier from distortion
when excessively strong signals are received.
The RF signals from the antenna connector are passed
through or bypass the "L" type attenuator (R1, R3). The sig-
nals are then applied to the RF filter circuit.
4-1-2 RF FILTER CIRCUIT
The applied signals pass through either the low-pass filter or
the high-pass filter circuits via the band switching diodes.
• RF signals below 50 MHz
The RF signals below 50 MHz are passed through the low-
pass filter (L1, L2, C7–C11) via the band switching diode
(D2). The filtered signals are applied to the HF RF circuit.
• RF signals above 50 MHz
The RF signals above 50 MHz are applied to the high-pass
filter (L172, C477, C478) after passing through the band
switching diode (D4). The filtered signals are then applied to
the VHF/UHF RF circuit.
4-1-3 HF RF CIRCUIT
The HF RF circuit amplifies the received signals within the
range 0.01–50 MHz and filters out-of-band signals.
The HF RF circuit consists of three low-pass filters, three
high-pass filters and one RF amplifier.
The filtered signals below 1.8 MHz from the RF filter circuit
are passed through the low-pass filter (L3, L4, C14–C16)
between the band swithing diodes (D6, D35), and are then
applied to the 1st mixer circuit (IC4) directly.
The 1.8–14.999 MHz signals pass through the low-pass fil-
ter (L6, L7, C21–C25) and high-pass filter (L8, L9,
C26–C30) between the band switching diodes (D3, D7), and
are then applied to the 1st mixer circuit after being amplified
at the RF amplifier (Q12).
• RF filter and amplifier circuits
ATT

CIRCUIT DESCRIPTION

0.01–1.8 MHz
0.01–50 MHz
1.8–15 MHz
AGC
LPF
15–30 MHz
30–50 MHz
50–150 MHz
50–1300 MHz
150–350 MHz
HPF
350–700 MHz
700–1300 MHz
The 15–29.999 MHz signals pass through the low-pass filter
(L10, L11, C33–C37) and high-pass filter (L11, L12,
C38–C42) between the band switching diodes (D90, D91),
and are then applied to the 1st mixer circuit via the RF ampli-
fier circuit (Q12).
The 30–49.999 MHz signals pass through the high-pass fil-
ter (L14, L15, C45–C49) between the band switching diodes
(D8, D5), and are then applied to the 1st mixer circuit via the
RF amplifier circuit (Q12).
• Filters
Receive freq.
SW diode
(MHz)
0.01–1.799 D6, D35
1.8–14.999 D3, D7
15.0–29.999 D90, D91
30.0–49.999 D8, D5
4-1-4 VHF/UHF RF CIRCUIT
The VHF/UHF RF circuit amplifies the received signals with-
in the range 50–1300 MHz and filters out-of-band signals.
The VHF/UHF RF circuit consists of 4 bands of filter circuits
with an RF amplifier for each.
The 50–149.999 MHz signals from the RF filter pass through
high-pass filter (D11, L17, C53–C55, D12, D82, D83, L18,
C57) via the band switching diode (D10), and are then
amplified at the RF amplifier (Q8) between the tunable
bandpass filters (D13, D80, L19–L21, D14, D81, L23–L25).
The filtered signals are applied to the 1st mixer circuit (IC4)
via the band switching diode (D15).
For improving the characteristic of the bandpass filter circuit,
the shift switch (Q31) shifts the cut off frequency of the high-
pass filter (D12, D82, D83, L18, C57). The shift switch (Q31)
is controlled by the VCO 1 signal from the CPU (IC21).
LPF
LPF
HPF
LPF HPF
HPF
RF Amp.
Tuned Tuned
Q8
BPF BPF
RF Amp.
Tuned Tuned
Q9
BPF BPF
RF Amp.
Tuned Tuned
Q10
BPF BPF
RF Amp.
Tuned Tuned
Q11
BPF BPF
4 - 1
Filter select
Components
signal
B0 L3–L5, C14–C17
B1 L6–L9, C21–C30
B2 L10–L13, C33–C42
B3 L14, L15, C45–C49
RF Amp.
Q12
To 1st mixer circuit