Agilent Technologies 4395A - Page 425

Calibration
for
Network
Measurement
A
ccuracy
Enhancement
Fundamentals-Characterizing
Systematic
Errors
One-P
ort Error
Model
In
a measurement
of
the
reection
coecient
(magnitude
and
phase)
of
an
unknown
device
,
the
measured data
diers
from
the
actual,
no
matter
how
carefully
the
measurement
is
made.
Directivity
,
source
match,
and
reection
signal
path
frequency
response
(tracking)
are
the
major
sources
of
error
(
Figure
A
-26
).
Figure
A
-26.
Sources
of
Error
in
a
Reection
Measurement
Measuring reection
coecient.
The
reection
coecient
is
measured
by
rst
separating
the incident
signal
(I)
from
the
reected
signal
(R),
then
taking
the
ratio
of
the
two
values
(Figure
A
-27
).
Ideally
,
(R)
consists
only
of
the
signal
reected
by
the
test
device
(S
11A
).
Figure
A
-27.
Reection
Coecient
Directivity Error
. However, all of the incident signal does not
always reach the unknown (see
Figure A
-28). Some of (I) may appear at the measurement
system input due to leakage through
the test set or other signal separation device
. Also
,some of
(I) may be reected by imperfect
adapters between signal separation and the measurement plane
. The vector sum of the
leakage
and miscellaneous reections is directivity
,E
DF
. Understandably
, the measurement is distorted
when the directivity signal combines vectorally with the actual reected signal from the
unknown, S
11A
.
Basic Measurement Theory A-49