Agilent Technologies 4395A - Page 427

Calibration
for
Network
Measurement
Figure
A
-30.
Reection
Tracking
E
RF
How
calibration
standards
are used
to quantify
these error
terms
.
It
can
be
shown
that
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"
that
makes
S
11A
=
0
and
essentially
measures
directivity
(
Figure
A
-31
).
\P
erfect
load"
implies
a
reection-free
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
LO
ADs
are
available
in
the
4296A
compatible
calibration
kits
.
Figure A
-31. \P
erfect Load" T
ermination
Because 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
\perfect load" (Figure A-32). In general, any termination having a return loss value greater
than the uncorrected system directivity reduces reection measurement uncertainty.
Basic Measurement Theory A-51