Icom IC-821H - 10 Mhz if CIRCUIT (MAIN UNIT); Demodulator Circuit (Main Unit); Squelch Circuit (Main Unit); Squelch Control Circuit

4-1-6 10 MHz IF CIRCUIT (MAIN UNIT)
The 10 MHz IF signal from the mixer circuit is passed
through the bandpass
filter (FI2 [Main], FI6 [Sub]) to sup-
press out-of-band signals. The filtered signal is amplified at
the IF amplifier (013 [Main], 098 [Sub]). The IF amplifier
provides 20 dB gain.
The
amplified signal is then applied to different circuits
depending
on the selected mode.
(1)
FM mode
The
signal is applied to an FM IF IC pin 16 (IC11 [Main] or
IC20 [Sub]).
(2) SSB and CW mode
The
signal is passed through a 10 MHz IF filter (FI1 /1 0.85
MHz [Main] or FI5/10.95 MHz [Sub]) or optional CW narrow
filters. The filtered signal is amplified at the IF amplifiers
(060-062 [Main] or 077. 0100. 0101 [Sub]) and then
applied to a demodulator circuit.
4-1-7 DEMODULATOR CIRCUIT (MAIN UNIT)
(1) FM mode
The
10 MHz IF signal from an IF amplifier (013 [Main] or
098 [Sub]) is applied to the mixer section of the FM IF IC
(IC11 [Main], IC20 [Sub). pin 16) and is mixed with a lO
signal (10.395 MHz [Main). 10.495 MHz [Sub]) to produce a
455
kHz IF signal. The lO signal is generated by the BFO
circuit (IC1 01 [Main], IC103 [Sub])
The FM detector circuit employs the quadrature detection
method. which uses a ceramic discriminator
(X2 [Main], X4
[Sub]) for phase delay to obtain a non-adjusting circuit.
The detected
signals are output from pin 9, and applied to
the
squelch control and center indication detector circuits.
etc.
(2) SSB and CW modes
The amplified signal from the IF amplifier circuit (062
[Main].
0101 [Sub]) is applied to the balanced mixer circuit
(IC14 [Main]. IC22 [Sub]) to demodulate into AF signals.
Demodulated
audio signals are output from pin 3, and
applied to the squelch control gate (IC16 [Main]. IC28
[Sub]) .
• AF circuit
IC14
Demodulator
[MAIN band]
BFO circuits (IC101 [Main] and IC103 [Sub]) generate BFO
signals for use in the balanced mixers.
• BFO frequencies
Mode
for MAIN band for SUB band
USB 10.8485 MHz 10.9485 MHz
lSB 10.8515 MHz 10.9515 MHz
CW 10.8483 MHz 10.9483 MHz
4-1-8 SQUELCH CONTROL CIRCUIT
(MAIN UNIT)
The demodulated AF signals from the balanced mixer
circuit or
FM IF IC are applied to the squelch control gate
(IC16 [Main), IC28 [Sub]). This consists of 4 analog
switches which are selected with a mode signal and squelch
control signal
from the CPU (DISPLAY unit, IC1) via the
expander
IC (IC8 [Main], IC24 [Sub]). The switched AF
signals are applied to the AF circuit.
4-1-9 SQUELCH CIRCUIT (MAIN UNIT)
(1) FM mode
A
squelch circuit cuts out AF signals when no RF signal is
received or the S-meter signal is lower than the [SOL]
control
setting level. By detecting noise components in the
AF signals, the CPU switches the squelch control gate.
Some noise components in the AF signals from pin 9 of the
FM IF IC (IC11 [Main], IC20 [Sub]) are applied to the active
filter section
(pin 10) of the FM IF IC. The filtered noise
components are output from pin
11 , and then applied to the
noise detector circuit
(D35 [Main], D55 [Sub]) for conversion
into
DC voltages. The voltages are then applied to the
buffer-amplifier (IC116 [Main], IC118 [Sub]).
The buffer-amplified
DC voltages are applied to the CPU
(DISPLAY unit. IC1) via the ADMNO [Main]/ADSNO [Sub]
line. The [SOL] level signal is also applied to the CPU via
the gate
control circuit (DISPLAY unit. IC11) as a reference
voltage for comparison with the noise signals. Also, an
S-meter signal is applied to the CPU from FM IF IC pin 13
(IC11 [Main], IC20 [Sub] via the meter amplifier (IC15a
[Main], IC23b [Sub]). The CPU compares these signals.
then outputs a control signal to the squelch control gate.
JACK board MSP SSP
AFMM
IC32
rnrn
11
------------------------
IC22 [SUB band]
7
Demodulator
AFMS
4-2