Icom IC-2820H Service Manual page 9

RF CIRCUITS
<Right band>
• 118−174 MHz
The received signals from the antenna connector ANT-1
(J1) are passed through two LPFs (L101, 104, 108, C342,
346; L88, 92, 96, C318, 326, 330), then applied to the
RF amplifier (Q37) via TX/RX switch (D59). The amplified
signals are passed through the attenuator (D70) and tuned
BPF (D54, 58), before being applied to another RF amplifi er
(Q32). The amplified signals are applied to the 1st mixer
(IC20) via the another tuned BPF (D40, 43) and RX switch
(D35).
• 375−550 MHz
The received signals from the antenna connector (J1) are
passed through the LPF (L101, 104, 108, C342, 346) and
HPF (L77, 80, C296, 297, 303, 308), then applied to the
RF amplifier (Q28) via TX/RX switch (D53, 61, 62). The
amplified signals are passed through the attenuator (D30)
and tuned BPF (D18, 20, 23, 26), before being applied
to another RF amplifier (Q19). The amplified signals are
applied to the 1st mixer (IC20) via the another tuned BPF
(D10, 12, 14, 16) and RX switch (D7).
• 810−1000 MHz
The received signals from the ANT-1 (J1) are passed
through the HPF (L102, 105, 107, 110, C337, 339, 340,
343, 347, 349) and RX switch (D22), then applied to the RF
amplifier (Q23). The amplified signals are passed through
the attenuator (R39, 40, 51), and applied to the another RF
amplifi er (Q18) to be amplifi ed again. The amplifi ed signals
are then passed through another attenuator (R14) and RX
switch (D9) before being applied to the 1st mixer (IC20).
• DEMODULATOR CIRCUITS (Right band)
R351
R_R5
Q63
R352
R358
AGC voltage to the 1st IF amplifier (Q75)
FM-demodulatedAF signals
from the D/A converter (IC8)
FM-demodulated AF signals
to the AF circuits
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AM DEMODULATOR CIRCUITS
R376
C471
0.01
180K
Q67
C477
R372
R388
C475
C476
NOISE FILTER
R375 R389
R_SQL
R_RSSI
4 - 3
1ST IF CIRCUITS
RX signals from the RF circuits are converted into the 46.35
MHz 1st IF signal by being mixed with LO signals from the
right band VCO (Q72, D89, 90; Q73, D87, 91, 92).
The converted IF signal from the 1st mixer (IC20) is passed
through the IF fi lter (IF6) to be fi ltered. The fi ltered IF signal is
applied to the 1st IF amplifi er (Q75) via the limiter (D100). The
amplifi ed 1st IF signal is applied to the IF IC (IC18, pin 16)
2ND IF AND DEMODULATOR CIRCUITS
IC15 is an IF IC which contains 2nd mixer, limiter amplifi er,
noise amplifi er, quadrature detector and RSSI circuit, etc. in
its package.
The 1st IF signal from the 1st IF amplifi er (Q75) is converted
into the 450 kHz 2nd IF signal by being mixed with tripled
reference frequency signal (45.9 MHz) from the PLL IC (IC14)
via the tripler (Q52). The converted 2nd IF signal is output
from pin 3, and passed through the ceramic filter (FI3 for
narrow mode, FI4 for wide mode) to remove sideband noise,
then applied to the IF IC from pin 5 again.
• FM DEMODULATOR
The fi ltered 2nd IF signal from pin 5 is amplifi ed at the limiter
amplifier, and FM-demodulated at the quadrature detector
circuit (X3). The demodulated AF signals are output from pin
9 and routed to the AF circuits via two AF switches (IC12
and IC16).
• AM DEMODULATOR CIRCUITS
In the AM mode, the 2nd IF signal from the FI3 is applied to
the AM-demodulator circuit (Q63, Q67). The demodulated
AF signals are routed to the AF circuits via two AF switches
(IC12 and IC16).
2nd IF FILTERS
C927 C926 D95
FI3
D93
C929 C928
FI4
D94
D96
C487 C495
C486
45.9 MHz 2nd LO signal
from the PLL IC (IC14)
IF IC (IC18)
1st IF signal from
the 1st IF amplifier(Q75)
X3
R_R5
R415
QUADRATURE DETECTOR