Icom ic-775 Service Manual page 13

Each
1st
LO
signal
(69.1115-99.0015
MHz)
enters the
RF
unit
from the
PLL
unit
via PI or P2.
The
LO
signals
are amplified
at
Q1
or
Q8,
filtered
by a
low-
pass
filter,
and
then,
applied
to
each
1
st
mixer.
1st IF
frequency
Mode
Frequency
SSB
69.0115
MHz
CW,
RTTY
69.0106
MHz
AM,
FM
69.0100
MHz
The
detected voltage
is
also applied
to
the noise blanker
AGO
circuit (Q52)
and
is
then
fed
back
to the noise ampli-
fiers
(Q47-Q49)
as a
bias voltage.
The
noise
AGO
circuit
prevents closure
of
the noise blanker gate
for
long periods
by non-pulse-type
noise.
The
time constant
of
the noise
blanker
AGO
circuit
is
determined by
R55 and C41
.
Q54-
Q56
expand
the blank width
when
[NB
WIDE]
is
turned
ON.
The [BLK-WIDTH]
control
adjusts the blank
width.
4-1-5 1st
IF
CIRCUIT
(RF UNIT)
The
signals
from the noise blanker gate are then applied
to
the
2nd
IF
circuit.
The
1st IF circuit
filters
and
amplifies the 1st IF signals.
The
1st IF signals
combined
at
L34 are applied
to
an
MCF
(Monolithic Crystal
Filter;
FI1) to
suppress out-of-band
signals.
The
levels of
the
converted
1st IF signals
are adjusted
at
PIN attenuators (D1, D3,
D4 and
D5, D7, D8) controlled
by
the
[BALANCE]
control.
The
signals are applied
to
a
1st IF
amplifier
(04 or
01
1)
and then
combined
at
L34.
The combined
signals
pass
through the
pair of
MCF's
(FI1)
and are amplified
at
015.
The
signals are applied to the
2nd mixer (IC34)
in
the IF unit via
P3
(IF unit J8).
The
AGC
voltage
is
supplied to
the
2nd
gate
of
Q15.
4-1-6
2nd
MIXER
CIRCUIT
(IF
UNIT)
The 2nd
mixer
circuit
mixes
the
amplified 1st IF signals
and
2nd
LO
signal (60.00
MHz)
to
convert the
1st IF to
a
2nd
IF.
The
1st IF signals
from the
RF
unit
are converted
to
9
MHz
2nd
IF
signals
at
the
2nd mixer
(IC34).
The 2nd
IF
signals
are
2nd
IF
frequency
applied to FI1 to
suppress
undesired signals
such as
the
2nd
LO
signal,
and
are
then applied
to
the noise
blanker gate
(D68-D71).
Mode
Frequency
SSB
9.0115
MHz
CW, RTTY
9.0106
MHz
AM,
FM
9.0100
MHz
4-1-7
NOISE
BLANKER
CIRCUIT
(IF
UNIT)
The
noise blanker
circuit
detects pulse type
noise,
and
turns
OFF
the signal line
when
noise appears.
The 2nd
IF
signals
from
FI1
are applied to
the noise blanker
gate
(D68-D71).
A
portion of
the signals from
FI1
are
amplified
at
the noise
amplifiers
(Q47-Q49), then detected
at
the noise detector (D75)
to
convert the noise
components
to
DC
voltages.
The
signals
are then applied
to
the noise blanker switch
(Q57-Q59).
At the
moment
the detected voltage exceeds
the
Q59's
or
Q58's threshold
level,
Q57
outputs a blanking
signal to
close the noise blanker gate
(D68-D71)
by apply-
ing
reverse-biased voltage.
Q58
and
Q59
are
activated
by
narrow-width pulses and high
level pulses, respectively.
The
[NB LEVEL]
control
adjusts the threshold
level of
Q58
and
Q59.
4-1-8
2nd
IF
CIRCUIT
(IF
UNIT)
The
2nd
IF
circuit
amplifies
and
filters
the
2nd
IF signals.
The
signals
passed
through the noise blanker
gate
(D68-D71)
are amplified
at
Q46
and
applied to
a 2nd
IF
filter
as
shown
in
the
following table.
2nd
IF
filters
Mode
Used
filter
Control
signal
CW/RTTY
wide,
SSB
med.,
AM
nar.
FL-80
(FI2)
2N24
CW/RTTY
med. FL-32A
(FI3)
2N05
CW/RTTY
nar.
Optional FL-1 01
2NCO
SSB
wide,
AM
med.,
FM
nar.
Optional FL-1
02
2NAO
SSB
nar.
Optional
FL-223
2NSO
AM
wide,
FM
med. Bypassed
2NTH
The
filtered or
bypassed
signals are then applied
to
the 3rd
mixer
circuit
through
a
buffer amplifier
(Q38).
4-1-9
3rd
MIXER
AND
3rd
IF
CIRCUITS
(IF
UNIT)
The
3rd
mixer
circuit
mixes
the
2nd
IF
signals
and
the 3rd
LO
signal to
obtain the 3rd IF (455 kHz)
signals.
The 2nd
IF signals
from
Q38
are applied
to
the 3rd mixer
(IC18
pin
6).
The
3rd
LO
signal from the
PLL
unit
is
applied
to
the 3rd
mixer (IC18
pin
8).
The 2nd
IF signals
are
converted
to 3rd IF signals.
The
3rd IF signals
are ampli-
fied at
Q39, and
then
applied to
one
of
the 3rd
IF
filters.
3rd IF
filters
Mode Used
filter
Control signal
CW/RTTY
wide,
SSB
med.,
AM
nar.
FL-96
(FI4)
3R26
CW/RTTY
med. FL-52A
(FI5)
3R05
SSB
wide,
AM
med.,
FM
nar.
CFW455HT
(FI6)
3R60
AM
wide,
FM
med.
CFW455E
(FI7)
3RFM
CW/RTTY
nar.
Optional
FL-53A
3RCO
SSB
nar.
Optional FL-222
3RSO
4-2