Icom IC-R20 Service Manual page 10

(3) HF-H (above 15 MHz, below 30 MHz) signals
RF signals (15–30 MHz) from an attenuator (D69) pass
through the low-pass filter (L123–L125 and C851–C853),
band switch (D73) and bandpass filter (L81–L84 and
C511–C520). The filtered signals pass through another
band switch (D64), and are then amplified at an RF amplifi-
er (Q505). The amplified signals are applied to the next
band switch (D72).
(4) A-VHF (above 118 MHz, below 175 MHz) signals
RF signals (118–175 MHz) from an attenuator (D69) pass
through the low-pass filter (L123–L125 and C851–C853),
band switch (D74), and are then applied to the pre-amplifier
(Q550) which is controlled by "A_VHF3V" signal via the
D148. The signals are amplified at the RF amplifier (Q14),
and pass through the bandpass filter (D1, D2, L1 and
C14–C16). The filtered signals are applied to the next band
switch (D25).
(5) 30–300M (above 30 MHz, below 300 MHz and except
A-VHF signals) signals
RF signals (118–175 MHz) from an attenuator (D69) pass
through the low-pass filter (L123–L125 and C851–C853)
and band switch (D75). The filtered signals pass through the
bandpass filter (L9, L10, L92, L133, C40–C43, C665 and
C666), and are then amplified at an RF amplifier (Q36). The
amplified signals are applied to the next band switch (D34).
(6) A-UHF (above 300 MHz, below 470 MHz) signals
RF signals (300–470 MHz) from an attenuator (D69) pass
through the high-pass filter (L126, L127 and C854–C858)
and band switch (D3). The filtered signals pass through the
low-pass filter (L3, L4, C22, C23 and C218), and are then
amplified at the pre-amplifier (Q551) which is controlled by
"A_UHF3V" signal via the D150. The amplified signals are
applied to the RF amplifier (Q35), and then pass through the
bandpass filter (D4, D5, L60, C58 and C59). The signals are
applied to the next band switch (D29).
• B-BAND CIRCUIT
(1) B-VHF (above 118 MHz, below 175 MHz) signals
RF signals (118–175 MHz) from an attenuator (D69) pass
through the low-pass filter (L123–L125 and C851–C853),
band switch (D74), and are then applied to the pre-amplifier
(Q550) which is controlled by "B_VHF3V" signal via the
D149. The signals are amplified at the RF amplifier (Q549),
and pass through the bandpass filter (D145, D146, L120,
C833 and C834). The filtered signals are applied to the next
band switch (D147).
(2) B-UHF (above 330 MHz, below 470 MHz) signals
RF signals (330–470 MHz) from an attenuator (D69) pass
through the high-pass filter (L126, L127 and C854–C858)
and band switch (D3). The filtered signals pass through the
low-pass filter (L3, L4, C22 and C23), and are then amplified
at the pre-amplifier (Q551) which is controlled by
"A_VHF3V" signal via the D151. The amplified signals are
applied to the RF amplifier (Q548), and then pass through
the bandpass filter (D141, D142, L113, C814 and C816).
The signals are applied to the next band switch (D143).
(3) 800M (above 470 MHz, below 833 MHz) signals
RF signals (470–833 MHz) from an attenuator (D69) pass
through the high-pass filter (L126, L127 and C854–C858)
and band switch (D11). The filtered signals pass through the
bandpass filter (D173, D174, L129 and L130), and are then
amplified at the RF amplifier (Q24). The amplified signals
pass through the bandpass filter (D175, D176 and L132).
The signals are applied to the next band switch (D32).
(4) 1G (above 833 MHz, below 1305 MHz) signals
RF signals (833–1305 MHz) from an attenuator (D69) pass
through the high-pass filter (L126, L127 and C854–C858)
and band switch (D13). The filtered signals pass through the
bandpass filter (L11–L13, L43, C5, C45–C51), and are then
amplified at the RF amplifier (Q26). The signals are applied
to the next band switch (D36).
4-1-3 DOWN CONVERTER CIRCUIT
(RF UNIT AND DOWNCONV BOARD)
The down converter circuit converts the signals within
1305–3305 MHz into 0.15 MHz to 1305 MHz signals.
The converted frequencies can calculate as follow.
F(x)=X–1001 (X is above 1305 MHz, below 2306 MHz)
F(x)=X–2002 (X is above 2306 MHz, below 3305 MHz)
NOTE: F(x) is converted frequency
X is receiving frequency
(Calculate example)
• Convert into 599 MHz when receiving 1600 MHz.
• Convert into 798 MHz when receiving 2800 MHz.
The RF signals from the limiter (D68) are applied to the
DOWNCONV board via the J1, pin 8 on the DOWNCONV
board. The signals pass through the band switch (DOWN-
CONV board; D1) and high-pass filter (DOWNCONV board;
L1, L2, C2–C4, C41 and C42), and are then amplified at the
RF amplifier (DOWNCONV board; IC2, pin 1). The signals
are mixed with 1001 MHz or 2002 MHz LO signal at the
mixer (DOWNCONV board; IC3, pin 1). The converted sig-
nals output from pin 6, and pass through the band switch
(DOWNCONV board; D2).
The converted signals are applied to the RF unit via J1,
pin 1 on the board. According to converted frequency, the
signals are applied to the each matched RF circuit.
4-1-4 1ST MIXER AND 1ST IF CIRCUITS (RF UNIT)
The 1st mixer circuit converts the received RF signals to a
fixed frequency of the 1st IF signal with a PLL output fre-
quency. By changing the PLL frequency, only the desired
frequency will pass through the bandpass filters at the next
stage of the 1st mixer.
• A-BAND CIRCUIT
The RF signals from the band switch (D25, D29, D34 and
D72) are amplified at the RF amplifier (IC19, pins 1, 4), and
are mixed with 1st LO signals at the 1st mixer circuit (IC1,
pin 1) to produce a 429.1 MHz 1st IF signal. The 1st IF sig-
nal is output from pin 6, and passed through the bandpass
filter (FI4) to suppress unwanted harmonic components. The
filtered 1st IF signal is applied to the 2nd mixer circuit.
The 1st LO signals are generated at the 1st A_VCO circuit
and are applied to the 1st mixer (IC1, pin 3) directly.
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