Transmitter Circuits; S-Meter Circuit (Main Unit); Microphone Amplifier Circuit; Modulation Circuit (Main Unit) - Icom IC-821H Service Manual

4-1-13 S-METER CIRCUIT (MAIN UNIT)
The S-meter circuit indicates the relative received signal
strength while receiving and changes depending on the
received signal strength.
(1) FM mode
Some of the amplified IF signal is applied to the S-meter
detector section in the FM IF IC (lC11 [Main), IC20 [Sub)) to
be converted into DC voltage. The converted signal is
output from pin 13 and applied to the meter amplifier circuit
(IC15a [Main), IC23b [Sub)). The amplified signal is then
applied to the CPU (DISPLAY unit, IC1) via the ADMSM
[Main)/ADSSM [Sub) line. The CPU then outputs S-meter
drive signals as a METO signal.
(2)
sse
and CW modes
A portion of the AGC control signal is applied to the S-meter
amplifier. The amplified signal is then applied to the CPU to
drive the S-meter.
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(DISPLAY AND MAIN UNITS)
The microphone amplifier circuit amplifies audio signals
from the microphone or ACC connector and then applies
them to the modulation circuit. One microphone amplifier
circuit is commonly used for both FM/SSB and VHF/UHF.
Audio signals from the microphone are amplified at the
variable controlled amplifier (VCA; DISPLAY unit, IC3)
where the amplifier gain is controlled with the [MIG) control
setting.
The mic limiter circuit (029) activates when the [COMP)
switch is turned ON to limit VCA output. The amplifier gain
is also controlled with the [COMP LEVEL) control in such
cases. The amplified audio signals are then applied to the
other microphone amplifier circuit (IC2a) in the MAIN unit.
Audio signals from the ACC connector is amplified at IC2b.
These amplified signals from IC2a or IC2b are applied to
the IDC or the balanced mixer circuit depending on oper-
ating mode.
• SIGNAL CONSTRUCTIONS IN SATELLITE MODE
Example UHF receive / VHF transmit
MAIN band: Rx SUB band: Tx/Rx
433.000.0
VHF UHF
I
I
I
lh~
145.000.0
!
MAIN
unit
I
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RF-B unit ~-t-""'\
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~.-.-.-.-.-.-.I
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4-2-2 MODULATION CIRCUIT (MAIN UNIT)
(1) FM mode
The amplified audio signals from IC2a are pre-emphasized
and limited at IC1 b and then passed through the splatter.
filter (IC1 a). The filtered signals are then applied to the FM
modulation circuit (022) via the FM deviation adjustment
pot (R29). Also, subaudible tone signals from the BFO
circuit (IC1 01) are applied to the FM modulation circuit via
the tone deviation adjustment pot (R64).
The FM modulation circuit changes the generating fre-
quency of the FM local oscillator (017, X1) to generate an
FM signal. The modulated IF signal is passed through the
transmit gain control pot (R136) and then applied to the
transmit IF amplifier circuit.
When 9600 bps mode is selected, audio signals from the
ACC connector bypass the amplifiers and are applied to 04
directly via the external modulation switch (MAIN unit,
IC114, pins 1, 7). In such cases, the modulation limiter
(IC17) cuts off the audio line when overmodulation is
detected.
(2)
sse
mode (MAIN unit)
The amplified audio signals from IC2a are mixed with BFO
signals at the balanced mixer circuit (IC7) to produce a 10
MHz IF signal. The mixed signal is still a DSB signal,
therefore, the mixed signal passes through bandpass filter
circuit (FI1) to suppress unwanted side band signals. The
filtered signal is applied to the transmit IF amplifier circuit.
Mode
Transmit IF signal
USB 10.8485 MHz
LSB 10.8515 MHz
CW 10.8491 MHz
4-2-3 CW KEYING CIRCUIT (MAIN UNIT)
Keying signals (EDOT and EDSH) from the [KEY) jack (J19)
are applied to the CPU (DISPLAY unit, IC1), and the CPU
outputs an EKEY signal to the CW break-in circuit (0234,
084).
The signal output from the CW break-in circuit is
applied to the BFO mute switches (0120, 0121) to cut the
BFO signal line while releasing. While keying, BFO signals
are applied to the balanced mixer, and a DC bias voltage is
applied to leak carrier signals .
"W....
[SUB VOL)