Transmitter Circuits - Icom IC-7800 Service Manual Addendum

3-1-9 AGC CIRCUIT
(RXPLL-A AND RXPLL-B UNITS AND DSP-A AND
DSP-B BOARDS)
The AGC (Automatic Gain Control) circuit reduces IF ampli-
fier gain and attenuates IF signal to keep the audio output at
a constant level.
• RX-A CIRCUIT (RXPLL-A UNIT AND DSP-A BOARD)
A part of signals from the compositor circuit (RXPLL-A unit;
IC1505, pin 8) are applied to the AGC detector circuit
(IC1652, D1653), and are then amplified at the AGC ampli-
fier (RXPLL-A unit; IC1653, pin 3). The amplified signals are
applied to the DSP-A board via the J921, pin 1 on the DSP-
A board as "AGCDET_A" line.
The amplified signals are applied to the amplifier (DSP-A
board; IC501, pin 2), and are then applied to the A/D con-
verter (DSP-A board; IC401, pin 25). The converted digital
signals are applied to the DSP IC (DSP-A board; IC1), and
then applied to the D/A converter (DSP-A board; IC771). The
converted analog signals are amplified at the AGC amplifier
(DSP-A board; IC501, pin 5), and are then applied to the
MAIN unit via the J921, pin 6 as "AGCOUT_A" line.
The amplified signals are applied to the RXPLL-A unit via
the J1701, pin 9 as "DAGC" line after being passed through
the J400, pin 8 on MAIN unit as "AGC1A" line. The applied
signals are amplified at the AGC amplifier (RXPLL_A unit;
IC1653, pin 5), and are then applied to the IF amplifier
(RXPLL-A unit; Q1403) as AGC signal.
• RX-B CIRCUIT (RXPLL-B UNIT AND DSP-B BOARD)
A part of signals from the compositor circuit (RXPLL-B unit;
IC1505, pin 8) are applied to the AGC detector circuit
(IC1652, D1653), and are then amplified at the AGC ampli-
fier (RXPLL-B unit; IC1653, pin 3). The amplified signals are
applied to the DSP-B board via the J921, pin 1 on the DSP-
B board as "AGCDET_B" line.
The amplified signals are applied to the amplifier (DSP-B
board; IC501, pin 2), and are then applied to the A/D con-
verter (DSP-B board; IC401, pin 25). The converted digital
signals are applied to the DSP IC (DSP-B board; IC1), and
then applied to the D/A converter (DSP-B board; IC771).
The converted analog signals are amplified at the AGC
amplifier (DSP-B board; IC501, pin 5), and are then applied
to the MAIN unit via the J921, pin 6 as "AGCOUT_B" line.
• DSP TRANSMITTER CIRCUIT
AF signal
MAIN unit
"ANVOX"
signal
IC341,
IC371
L+
A/D
converter
IC301
(IC401)
L
Low-pass
filter
R+
LPF
R
The amplified signals are applied to the RXPLL-B unit via
the J1701, pin 9 as "DAGC" line after being passed through
the J400, pin 8 on MAIN unit as "AGC1B" line. The applied
signals are amplified at the AGC amplifier (RXPLL_B unit;
IC1653, pin 5), and are then applied to the IF amplifier
(RXPLL-B unit; Q1403) as AGC signal.

3-2 TRANSMITTER CIRCUITS

3-2-1 MICROPHONE AMPLIFIER CIRCUIT (MAIN UNIT)
The microphone amplifier circuit amplifies microphone audio
signals to a level needed for the DSP circuit.
Audio signals from [MIC] connector (MIC board; J1, pin 1)
are amplified at the audio amplifier section in IC821 (pin 2),
then applied to the VCA section. The gain controlled signals
are output from (IC821, pin 4) and passed through the ana-
log switch (IC201, pins 1, 7). The signals pass through the
analog switch (IC460, pins 1, 6), and are then applied to the
DSP circuit after being passed through the limiter amplifier
(IC230, pins 1, 3) as the "AFIN" signal.
The VCA section in IC821 controls microphone input gain
according to [MIC GAIN] control level using the MIGV signal
coming from the main CPU via the I/O expander (IC330,
pin 2).
3-2-2 DSP TRANSMITTER CIRCUIT (DSP-TX BOARD)
The DSP (Digital Signal Processor) circuit enables digital IF
filter, digital noise reduction, digital PSN (Phase Shift
Network)/Low Power/Phase demodulation, digital automatic
notch, and etc.
The AF signals from the limiter amplifier (MAIN unit; IC230,
pin 1) are then applied to the buffer amplifiers (IC301; pins
2, 6, IC341, pins 2, 6, IC371, pins 2, 6) to convert to bal-
ancing signal. The signals are applied to the A/D convertor
(IC401, pins 4, 5).
The converted digital signals are applied to the DSP IC (IC1)
to do filtering, modulate and so on. The modulated signals
are applied to the D/A converter (IC551) to convert into a
36 kHz IF signal.
DSP-TX unit
D/A
DSP IC
converter
(IC1)
(IC551)
3 - 5
IC601,
IC651
IC701
36 kHz TX IF signal
to the MAIN unit
Low-pass
filter