Philips PM 5390 - Coarse Attenuator Setting; Control, Fine; Level Control, Fire and Coarse Attenuation; RF-Unit 1, Pin Diode Attenuator

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