Dsp Circuits - Icom ic-775 Service Manual

4-6
DSP
CIRCUITS
The
DSP
(Digital
Signal Processor) unit
enables
digital
noise reduction, digital
PSN
(Phase
Shift
Network)
modu-
lation/demodulation,
digital
AF
automatic
notch,
digital
LPF
and HPF,
ultra-narrow
CW
filter
and
digital
automatic
APF
(Audio
Peak
Filter).
4-6-1
DSP RECEIVING
(DSP UNIT,
IC-775DSP
only)
When
receiving with
the
DSP
function
ON,
detected
AF
signals
are applied
to
the
DSP
unit via
the
AF
selector
(IF
unit
IC17).
When
receiving with
PSN
demodulation
in
SSB/
CW/RTTY
modes, the product detector
(IF unit
IC14) func-
tions
as a
5th
mixer
to
obtain
a 15 kHz
IF signal.
The
IF
signal
is
applied to
the
DSP
unit (J1
pin 8) via
IC17
(IF unit)
instead of
AF
signals.
The
AF
or IF
signals
pass through the analog switch (IC3
pins 1,15)
and
are applied to
the
A/D
converter
(IC8).
The
digital
signal
is
demodulated
(5th IF signal)
by the
PSN
method
at
IC14 and
digitally
manipulated
at
IC15.
The
resulting digital
signal
is
then converted
into
an analog
signal at
the
D/A converter
(IC9).
The
converted signal
passes through the
limiter-amplifier
(IClOa)
and an analog switch (IC3 pins
13,
14).
The
signal
is
amplified
at
the
limiter-amplifier (IC4a),
passes through
the active
low-pass
filters
(IC5a,
IC5b)
and
is
then applied
to
the switched capacitor
filters
(IC6, IC7).
The
cut-off
frequencies
of
IC6
and IC7
can be programmed.
The
resulting signal
passes through
IClOb and IC3
(pins
3,
4)
and
is
applied
to
the
AF
selector (IC16)
in
the
IF
unit.
4-6-2
DSP TRANSMITTING
(DSP
UNIT,
IC-775DSP
only)
When
transmitting with the
DSP
function
ON, microphone
signals are applied
to
the
DSP
unit (J1 pin
4) via the
AF
selector (IF unit 109).
The
AF
signals
pass through the analog switch (IC3 pins
12,
14),
the limiter-amplifier (IC4a),
the active
low-pass
filters
(IC5a,
IC5b)
and
are then applied
to
the switched
capacitor
filters
(IC6, IC7).
The
cut-off
frequencies
of
IC6
and IC7
can be programmed.
The
resulting
AF
signal
passes through
IC1
Ob and
IC3
(pins 4/5,
2/1
5)
and
is
then
converted
into
a
digital
signal at
the
A/D
converter
(IC8).
The
digital signal
is
modulated by
the
PSN
method
at IC1
4
and
digitally
manipulated
at 1015.
The
resulting digital
signal
is
then converted
into
an analog
signal at
the
D/A
converter
(109).
The
converted signal
passes through the
limiter-amplifier
(1010a),
filtered
at
the
active
high-pass
filter
(1011),
ampli-
fied at
104b
and
is
applied
to
the
AF
selector (109)
in
the
IF
unit.
4-13