Icom ic-775 Service Manual page 14

The
filtered
signals
are amplified
at
Q12, and then applied
to
the 4th
mixer
or
FM
detector
circuit.
4-1-10 4th
MIXER
AND
4th
IF
CIRCUITS
(IF
UNIT)
The
4th
mixer
circuit
mixes the 3rd
IF
signals
and
the 4th
LO
signal
to
obtain the 4th
IF
(10.6950
MHz)
signals.
The
3rd IF
signals
from
Q12
are applied
to
the 4th mixer
(IC13
pin
6),
except
in
FM
mode, and
are
mixed
with the 4th
LO
signal to
be converted
into
4th IF signals.
The
4th IF
signals
are applied to
the
IF
notch
filter
(XI), filtered at
FIS
and
then amplified at
Q13
and Q14.
R198
sets the
total
gain.
The
4th IF
signals are applied to
the
AM
detector
circuit
via
the
buffer-ampiifier (Q15) or
product detector (IC14
pin
6).
IC14 functions
as the 5th
mixer
when
the
DSP
(Digital
Signal
Processor) receive
function
is
turned
ON.
4-1-11
TWIN
PBT
CIRCUIT
(IF
UNIT)
The
PBT
(PassBand Tuning)
circuit shifts
the center
fre-
quency
of IF
signals
to
electronically
narrow
the
passband
width.
The
IC-775/DSP has
2
PBT
circuit.
The
twin
PBT
circuit shifts
the
2nd and
3rd IF within
±
1.5
kHz.
As
a
result,
the
2nd and
3rd
IF
are
shifted
from the
center frequencies
of
the
2nd and
3rd
IF
filters.
This
means
2nd
or
3rd
IF
signals
do not
pass through the center
of
the
2nd
or 3rd IF
filters
because
the
passband
width
is
fixed
in
the
2nd
or
3rd
IF
filter.
Therefore, the overlap
of
the
2nd/3rd or 3rd/4th IF
filter
appears
to
be narrowed. Since
the 3rd
and
4th
LO frequencies are
also shifted
the
same
value as the
2nd and
3rd IF
shifts,
frequencies are cor-
rected at the detector.
In
the
IC-775/DSP, the 1st
LO
frequency
is
shifted to
change
the
2nd
IF
because
a
fixed
2nd
LO
frequency
(60
MHz)
is
used.
The
1st IF
filter
(RF
unit
FM) and
crystal
filter
(IF unit FI1)
have 15
kHz passband
widths
and do
not affect
PBT
operation.
4-1-12
IF
NOTCH
CIRCUIT
(IF
UNIT)
The
IF
notch
circuit
attenuates
a
specified
audio frequency
in
the
IF
section to
increase readability of
desired
signals.
The
IF signals
from
the 4th
mixer (1013) are appiied
to
the
IF
notch
circuit
(XI).
The
crystal
filter
has excellent
cut-off
frequency characteristics
and attenuation
of
more
than
45
dB.
The
IF
notch frequency
is
controlied
by
shifting
the 4th
IF to
obtain the
same
attenuation characteristics within
the
IF
band
width.
The
4th IF
frequency can be adjusted with
the
[NOTCH]
control.
When
the
[NOTCH]
switch
is
turned
OFF,
the 4th
IF
signals
pass through XI out
of
the notch
filtering
frequency. There-
fore,
the
IF
notch
circuit
is
substantially
bypassed by
the 4th
IF signals.
4-1-13
AGC
CIRCUIT
(IF
UNIT)
The
AGC
(Automatic Gain Control)
circuit
reduces
IF
ampli-
fier
gain
to
keep the audio output
at
a constant
level.
The
receiver gain
is
determined by the voltage
on
the
AGC
line
(Q17
collector).
Q26
supplies
minus
voltage to the
AGC
line
and
sets
the receiver gain with the [RF] control.
The
4th IF signal
from
the buffer amplifier (Q15)
is
detected
at
the
AGC
detector (DIO)
and
is
then
applied to
the
DC
amplifier (Q17).
-5 V
is
applied
to
the
Q17
emitter to
activate the
AGC
line
using
minus
voltage.
When
receiving strong signals,
the detected voltage
in-
creases and
the
AGC
voltage
decreases
via
the
DC
ampli-
fier
(Q17).
As
the
AGC
voltage
is
used
for
the bias voltage
of the IF amplifiers
(Q13,
Q14, Q39,
Q46
and
RF
unit Q15),
IF amplifier gain
is
decreased.
The
AGC
voltage
is
also
applied
to
the
PIN
attenuator
(D42)
in
the
RF
unit to
attenuate the received signals before the 1st mixers.
When
the strong signal
disappears, the
AGC
line
voltage
is
released
by
C146-C148 and
R222
while
in
SSB,
CW
and
RTTY
mode.
While
In
AM
or
FM
mode, Cl 45
or
R221
is
connected
in
parallel to
obtain appropriate
AGC
character-
istics,
respectively.
AGC
circuit
*Qc
AM
FM
OFF mod* mode
AGC
line
4-1-14
S-METER
CIRCUIT
(IF
UNIT)
The
S-meter
circuit
indicates
the
relative
received signal
strength while receiving by
utilizing
the
AGC
voltage
which
changes depending on the received
signal strength.
The
AGC
bias voltage (time
constant
line)
is
applied
to
a
differential amplifier
(IC7b
pin 6)
where
the difference be-
tween
the
AGC
and reference voltages
is
detected.
The
resulting
S-meter
signal
passes
through the
meter
switching
circuit
(IC6 pins
3,
13)
via the
"SQLV"
signal
line.
The S-meter
signal
is
applied to the
main CPU, sub
CPU,
and
then
to
the
S/RF meter on
the
front
panel.
The
S-meter
signal
(SQLV)
is
also applied
to
the
main
CPU
(MAIN
unit
IC12
pin 93) for
S-meter squelch
detection.
4-1-15
SQUELCH
CIRCUIT
(MAIN,
IF
AND
SW-A
UNITS)
4-3
The
squelch
circuit
mutes audio output
when
the S-meter
signal
is
lower.
than the [SQL] control setting
level.