HP 8753D - Page 356

Figure
6-53. Typical
Measurement Set
up
If
the device
measurement
requires
bias
,
it
will
be
necessary
to
add
external
bias
tees
between
the
xed attenuators
and
the
xture
.
The
internal
bias
tees
of
the
analyzer
will
not
pass
the
bias
properly through
the
external
xed
attenuators
.
Be
sure
to
calibrate
with
the
external
bias
tees
in place
(no
bias
applied
during
calibration)
to
remove
their
eect
from
the
measurement.
Because
the
bias
tees
must
be
placed
after
the
attenuators
,
they
essentially
become
part
of
the
xture
.
Therefore
,
their
mismatch
eects
on
the
measurement
will
not
be
improved
by
the
attenuators
.
Although
the
xed
attenuators
improve
the
raw
mismatch
of
the
network
analyzer
system,
they
also
degrade
the
overall
measurement
dynamic
range
.
This
eective
mismatch
of
the
system
after
calibration
has
the
biggest
eect
on reection
measurements
of
highly
reective
devices
.
Likewise
,
for
well
matched devices
,the
eects of
mismatch
are
negligible
.
This
can
be
shown
by
the
following
approximation:
Reection
magnitude
uncertainty
=
E
D
+
E
R
S
11
+
E
S
(S
11
)
2
+
E
L
S
21
S
12
Transmission
magnitude
uncertainty
=
E
X
+
E
T
S
21
+
E
S
S
11
S
21
+
E
L
S
22
S
21
where:
E
D
= eective directivity
E
R
= eective reection tracking
E
S
= eective source match
E
L
= eective load match
E
X
= eective crosstalk
E
T
= eective transmission tracking
S
xx
=S-parameters of the device under test
Application and Operation Concepts 6-89