HP 8753D - Page 323

Figure
6-35.
Reection
Tracking
E
RF
These
three
errors
are mathematically
related to
the actual
data, S
11A
,
and
measured
data,
S
11M
,
by
the
following
equation:
S
11
M
=
E
D
F
+
(
S
11
A
E
R
F
)
(1
0
E
S
F
S
11
A
)
If the
value of
these
three
\E"
errors
and
the
measured
test
device
response
were
known
for
each frequency
,
the
above
equation
could
be
solved
for
S
11A
to obtain
the
actual
test
device
response.
Because
each
of
these
errors
changes
with
frequency
,
their
values
must
be
known
at each
test
frequency
.
These
values
are
found
by
measuring
the
system
at
the
measurement
plane
using
three
independent
standards
whose
S
11A
is
known
at
all
frequencies
.
The
rst
standard
applied
is
a
\perfect
load,"
which
makes
S
11A
=
0
and
essentially
measures
directivity
(see
Figure
6-36
).
\P
erfect
load"
implies
a
reectionless
termination
at
the
measurement
plane
.
All
incident
energy
is
absorbed.
With
S
11A
=
0
the
equation
can
be
solved
for
E
DF
,
the
directivity
term.
In
practice
,
of
course
,
the
\perfect load"
is
dicult
to
achieve
,
although
very
good
broadband
loads
are
available
in
the
HP
8753D
compatible calibration
kits.
Figure 6-36. \P
erfect Load" T
ermination
Since the measured value for directivity is the vector sum of the actual directivity plus
the actual reection coecient of the \perfect load," any reection from the termination
represents an error. System eective directivity becomes the actual reection coecient of the
near \perfect load" (see Figure 6-37). In general, any termination having a return loss value
greater than the uncorrected system directivity reduces reection measurement uncertainty.
6-56 Application and Operation Concepts