Agilent Technologies 4395A - Page 417

Calibration
for
Network
Measurement
temperature
drift, and
other physical
changes
in
the
test
setup
between
calibration
and
measurement.
The
resulting measurement
is
the
vector
sum
of
the
DUT
response
plus
all
error
terms.
The
precise
eect of
each
error
term
depends
upon
its
magnitude
and
phase
relationship
to
the
actual
test device
response
.
In
most
high
frequency
measurements
the
systematic
errors
are
the
most
signicant
source
of
measurement
uncertainty
.
Because
each
of
these
errors
can
be
characterized,
their
eects
can
be
eectively
removed
to
obtain
a
corrected
value
for
the
test
device
response
.F
or the
purpose
of
vector
accuracy
enhancement
these
uncertainties
are
quantied
as
directivity
,
source
match,
load
match,
isolation
(crosstalk),
and
frequency
response
(tracking).
Each
of
these
systematic
errors
is
described
below
.
Random
and
drift
errors
cannot
be
precisely
quantied,
so
they
must
be
treated as
producing a
cumulative
uncertainty
in
the
measured
data.
Directivity
Normally
a
device
that
can
separate
the reverse
from
the
forward
traveling
waves
(a
directional
bridge
or
coupler)
detects
the
signal
reected from
the
DUT
.
Ideally
the
coupler
would
completely
separate
the
incident
and
reected
signals
,and
only the
reected signal
would
appear
at
the
coupled
output
(
Figure
A-23
-a).
Figure A
-23. Directivity
However, an actual
coupler is not perfect (Figure A
-23-b). A small amount of the incident
signal appears at the coupled
output due to leakage as well as to reection from the
termination in the coupled arm. Also
, reections from the main coupler output connector
appear at the coupled output, adding uncertainty to the signal reected from the device. The
gure of merit for how well a coupler separates forward and reverse waves is directivity. The
greater the directivity of the device, the better the signal separation. Directivity is the vector
sum of all leakage signals appearing at the analyzer receiver input due to the inability of the
signal separation device to separate incident and reected waves, and to residual reection
eects of test cables and adapters between the signal separation device and the measurement
Basic Measurement Theory A-41