Icom IC-775 - 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.