Agc Circuit; Transmitter Circuits; Microphone Amplifier Circuit; Dsp Transmitter Circuit - Icom IC-7800 Service Manual

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
amplifier 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
amplifier (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
converter (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
amplifier (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
converter (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"

• DSP TRANSMITTER CIRCUIT

AF signal
IC301
MAIN unit
"ANVOX"
signal
the A/D
IC341,
IC371
L+
A/D
converter
(IC401)
L
Low-pass
filter
R+
LPF
R
line. 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
analog 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 balancing 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