HP 8753D - Page 429

Fixtures
Fixtures
are
needed
to
interface
non-coaxial
devices
to
coaxial
test
instruments.
It may
also be
necessary
to
transform
the
characteristic
impedance
from
standard
50 ohm
instruments to
a
non-standard
impedance
and
to
apply
bias
if
an
active device
is being
measured.
F
or
accurate
measurements
,
the
xture
must
introduce minimum
change to
the test
signal, not
destroy
the
test
device
,
and
provide
a
repeatable
connection
to the
device.
Hewlett-P
ackard
oers
several
xtures
for
TO
cans
, stripline
, and
microstrip devices
.
Refer
to
Chapter
11,
\Compatible
P
eripherals
."
If
Y
ou
W
ant
to
Design
Y
our Own
Fixture
Ideally
,
a
xture
should
provide
a
transparent
connection between
the test
instrument and
the
test
device
.
This means
it should
have no
loss or
electrical
length
and
a
at
frequency
response
,
to
prevent
distortion of
the actual
signal. A
perfect match
to
both
the
instrument
and
the
test
device
eliminates reected
test signals
. The
signal should
be
eectively
coupled
into
the
test
device
,
rather
than leaking
around the
device
and
resulting
in
crosstalk
from
input
to
output.
Repeatable
connections
are
necessary
to
ensure
consistent
data.
Realistically
,
it
is
impossible
to
build
an
ideal
xture
,
especially
at
high
frequencies
.
However
,
it
is
possible
to
optimize
the
performance
of
the
test
xture
relative
to
the
performance
of
the
test device
.
If
the
xture's
eects
on
the
test
signal
are
relatively
small
compared
to
the
device's
parameters
,
then
the
xture's
eects
can
be
assumed
to
be
negligible
.
F
or
example
,
if
the
xture's
loss
is
much
less
than
the
acceptable
measurement uncertainty
at
the
test
frequency
,
then
it
can
be
ignored.
6-162 Application and Operation Concepts