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3-13
3.5.4.
Level
control, fine
and
coarse attenuation
The
output
signal
of the
main
oscillator
VCO
1
is
fed to the
automatic
level
control,
where
the
RF
leve^
is
adjusted
to
-1
dBm
±1
dB
at
mixer 851,
pin
4.
The
RF
signal
is
detected
by
diode
410
and
applied
to
amplifiers
302/301.
The
dc
voltage
at
301.6
(about 4.5
V
±0.2 V)
controls pin
diodes 411,
412,
413.
Amplitude/video modulation
can be
added
after this stage.
The
appropriate
signal
is
applied
from
unit
3 to
mixer 851,
pin
2.
The
final
section provides fine
and
coarse attenuation
in
two programmable
stages.
Fine adjustment
is
carried
out by
the pin diode attenuator
on
RF-unit
1,
providing
a
continuous
range
from 0
to
20
dB
in
1
dB
steps.
Additional
temperature compensation
is
realized
by
NTC
resistor
740.
Coarse
adjustment
is
achieved
by
a
100
dB
attenuator
In
10
dB
steps
at
the
RF
output.
The 0
to
9
dB
range
of the
pin
diode
attenuator
is
used
for
level
setting
from
-7
dBm
to
-117
dBm
(PM 5390
S:
-M3
dBm
to
-117
dBm),
while the
10
to
20 dB
range
is
active
from
-1
17
to
-127
dBm
only.
The
circuit
diagram
of the pin
diode
attenuator
and the
applied control voltage
VR
2
(exponential
characteristic)
is
shown
in fig.
9a PIN
diodes have a
resistance characteristic for
HF
signals.
The
value
of the
HF
resistance
depends on
the
dc
bias
current.
For adjustment procedure
the
characteristic
of
the
control voltage
VR
2
may
be
changed
by
solder
joint
E
on
unit
1
Complete
control of the coarse
and
fine
attenuation
is
achieved
by
output
lines
B7,
CO
—
C7
of
port
2
and BO
-
B2,
port
3,
on
control unit
2.
I
Fig.
9a
RF-unit
1,
pin
diode attenuator
Fig.
9b
Coarse
attenuator
setting
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