3
Chapter 2
Theory of Operation
The
Series
8320
may
be
used
for
the
isolation
of
power
sources
up
to
their
maximum
power
rating
and
for
low
level
monitoring.
The
low
power
value
obtained
at
the
output
of
the
attenuator
can
easily
be
fed
into
an
oscilloscope
or
terminated
in
a
small
RF
load
resistor.
Attenuator Construction
The
Series
8320
Attenuators
are
symmetrical
“T”
pads,
with
the
power
distribution
on
the
legs
being
different.
The
value
of
the
resistance
on
each
leg
varies;
therefore,
according
to
the
power
it
is
to
absorb.
On
the
input
resistance
element,
a
proportionately
larger
resistor
is
of
course
required
for
its
much
greater
power
dissipation.
A
“T”
configuration
is
used
to
provide
equal
input
and
output
impedances
for
the
50
ohm
transmission
line
attenuation.
The
input
resistor
is
joined
to
the
“T”
leg
joint
in
an
exponentially
tapered
housing
to
provide
a
linear
reduction
in
surge
impedance
directly
proportional
to
the
distance
along
the
resistor.
The
output
resistor
is
enclosed
in
a
housing
designed
to
return
the
attenuator
to
the
characteristic
impedance
of
50
ohms.
This
arrangement
produces
a
uniform
and
practically
reflection
‐
less
attenuation
characteristic
over
the
stated
frequencies
of
the
attenuator.
This
system
of
film
‐
on
‐
ceramic
cylindrical
resistors
immersed
in
a
dielectric
coolant
constitutes
the
RF
Assembly.
The
dielectric
constant
of
the
cooling
fluid
and
the
tapered
input
and
output
resistor
housings
provide
the
proper
electrical
characteristics
for
the
coaxial
line
attenuation
throughout
the
internal
circuitry.
Cooling
The
dielectric
coolant
is
carefully
chosen
for
its
desirable
dielectric
properties,
to
which
the
diameters
of
the
resistors
and
housings
are
matched,
and
for
its
high
thermal
stability
characteristics.
The
coolant
used
is
10C
Transformer
Oil,
P/N
5
‐
030.
Model
8321
requires
approximately
1
pint
(0.47
liter)
Model
8322,
0.7
gallons
(2.65
liter)
Model
8323,
0.35
gallons
(1.3
liter)
Model
8325,
0.9
gallons
(3.4
liter).
