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4-1-5 DSP RECEIVER CIRCUIT (MAIN AND DSP
UNITS)
The DSP (Digital Signal Processor) circuit enables digital
IF filter, digital noise reduction, digital PSN (Pulse Shift
Network), phase demodulation, digital automatic notch, and
etc.
The 3rd IF signal is applied to the IF amplifier (MAIN
unit; IC1002, pin 5) after being passed through the low-
pass filter (MAIN unit; IC1002, pins 3, 1). The amplified 12
kHz 3rd IF signal is amplified at the differential amplifiers
(IC9651a/b), and is then applied to the A/D convertor
section in the CODEC IC (IC9501) on the DSP board. At
the same time, the converted signal is level-shifted 5 V to 3.3
V in the IC (IC9501).
The level-shifted signal is applied to the DSP IC (IC9301)
for the digital IF filter, demodulator, automatic notch and
noise reduction, etc.
The output signal from the DSP IC is applied to the D/A
converter section in the CODEC IC (IC9501) to convert into
the analog audio signals. Also the signals are level-shifted
3.3 V to 5 V at the level converter section in the IC (IC9501).
The level-shifted audio signals are passed through the
active filter (IC9701a), and then applied to the MAIN unit
via J9901 (pin 22) as the "DSP02" signal.
4-1-6 AGC CIRCUIT (DSP AND MAIN UNITS)
The AGC (Automatic Gain Control) circuit reduces IF
amplifier gain and attenuates IF signal to keep the audio
output at a constant level.
The receiver gain is determined by the voltage on the
AGC1 line from the DSP unit. The D/A converter for the
AGC (IC9102) supplies control voltage to the AGC1 line
and sets the receiver gain with the [RF/SQL] control.
• DSP CIRCUIT
IC9651b/a
3rd IF
Differential
converter
signal
DSPI1
(12 kHz)
MAIN UNIT
DSP UNIT
• AGC CIRCUIT
DSPI1
DSP
UNIT
CODEC IC (IC9501)
A/D
Level
converter
converter
IC1002
Q501
IF
amp.
IC1101
2
1
AGC1
AGC
AGC2
6
7
AGC
The 3rd IF signal from the CODEC IC (IC9501) is detected
at the AGC detector section in the DSP IC (IC9301).
The output signal from the DSP IC is level-shifted at the
level converter (IC9101) and applied to the D/A converter
(IC9102). The AGC voltage is amplified at the buffer
amplifier section in the IC9102 and applied to the MAIN
unit to control the AGC1 line.
When receiving strong signals, the detected voltage
increases and the AGC1 voltage decreases. As the AGC1
voltage is used for the bias voltage of the IF amplifiers (MAIN
unit; Q203, Q401, Q501), IF amplifier gain is decreased.
4-1-7 AF AMPLIFIER CIRCUIT (DSP AND MAIN
UNITS)
The AF amplifier amplifies the audio signals to the suitable
driving level for the speaker.
The AF signals from the DSP unit are applied to the AF
amplifier (MAIN unit; IC1651, pin 3) via the "DSPO2" line.
The signals are amplified at the other AF amplifier (IC1651,
pin 5) after being passed through the low-pass filter. The
signals are applied to the AF mute switch (IC1602, pin 2),
and then amplified at the AF power amplifier (IC1601, pin
1). The amplified signals are applied to the speaker (SP-26)
after being passed through the speaker jack (J1451) via
the "AFO" signal.
IC9301
DSP IC
D/A
converter
Q401
IF
IF
amp.
amp.
4 - 2
IC9701a
Active
filter
Level
converter
From the
antenna
Q203
IF
amp.
DSPO1
AF
signals
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