Icom IC-E2820 Service Manual page 8

1ST IF CIRCUITS
RX signals from the RF circuits are converted into the 38.85
MHz 1st IF signal by being mixed with LO signals from the
left band VCO (Q111, D145−147).
The converted IF signal from the 1st mixer is passed through
the IF fi lter (FI5) to be fi ltered. The fi ltered IF signal is applied
to the 1st IF amplifi er (Q66) via the limiter (D88). The amplifi ed
1st IF signal is applied to the IF IC (IC15, pin 16)
2ND IF AND DEMODULATOR CIRCUITS (Fig. 2)
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 (Q66) is converted
into the 450 kHz 2nd IF signal by being mixed with tripled
reference frequency signal (38.4 MHz) from the PLL IC (IC41)
via the tripler (Q105). The converted 2nd IF signal is output
from pin 3, and passed through the ceramic filter (FI1 for
narrow mode, FI2 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. The demodulated AF signals are output from pin 9
and routed to the AF circuits via two AF switches (IC11 and
IC13).
• AM DEMODULATOR CIRCUITS
In the AM mode, the 2nd IF signal from the FI2 is applied to
the AM-demodulator circuit (Q55, Q57). The demodulated
AF signals are routed to the AF circuits via two AF switches
(IC11 and IC13).
• 2ND IF AND DEMODULATOR CIRCUITS (LEFT BAND)
DEMODULATOR CIRCUITS (Left band)
AM DEMODULATOR CIRCUITS
L_R5
R306
R310
AGC voltage to the 1st IF amplifier (Q66)
FM-demodulatedAF signals
from the D/A converter (IC8)
FM-demodulated AF signals
to the AF circuits
R328
Q55
R307 R317
C418
C419
NOISE FILTER
R308
L_SQL
L_RSSI
SQUELCH CIRCUITS
• NOISE SQUELCH
A portion of FM-demodulated AF signals from the IF IC
(IC15, pin 9) are level-adjusted by D/A converter (IC8), and
passed throuhgh the noise fl tier (IC15 and some R and C) to
be fi ltered noise components (30 kHz and above signals) in
the AF signals. The fi ltered noise components aree detected
in the IC15 and output from pin 13, then applied to the CPU
(IC25, pin 100) as "L_SQL" signal.
Then the CPU outputs "L_AF_MUTE" signal from pin 82 to the
speaker mute switch (Q102), according to the "L_SQL" signal
level. Thus the AF line is connected to the GND to turn the AF
output OFF.
• CTCSS/DTCS
CTCSS/DTCS signals in the demodulated AF signals from
the AF switch (IC13) are passed through the tone fi lter (Q41)
. The fi ltered CTCSS/DTCS signals are applied to the CPU
IC25, pin 70) as "L_DTCSIN" signal. The CPU (IC25) com-
pares the applied signal and the set CTCSS/DTCS, then out-
puts control signal as same as "NOISE SQUELCH."
• DTMF
DTMF signals in the demodulated AF signals from the AF
switch (IC13) are passed through two AF switches (IC57
and IC58), then applied to the DTMF decoder (IC56) to be
decoded.
AF CIRCUITS
The AM/FM-demodulated AF signals from the AF switch
(IC11) are passed through the AF filter (Q47). The filtered
AF signals are applied to the electric volume (IC33) to be
adjusted its level. The level-adjusted AF signals are applied
to the dual AF power amplifiier (IC38) to obtain AF output
power level, then applied to the internal (CHASSIS; SP1)
or an external speaker via external speaker jack (J7).
If an external speaker is connected to the J8, the level-
adjusted AF signals from the electric volume (IC33) are
applied to the connected speaker.
2nd IF FILTERS
D82 C932
C427
D83
C933
Q57
C429
QUADRATURE DETECTOR
R315
C435
4 - 2
FI1
C437
C930 D84
I O
FI2
D85
C931
I O
38.4 MHz 2nd LO signal
from the PLL IC (IC41)
IF IC (IC15)
1st IF signal from
the 1st IF amplifier(Q66)