HP 8753D - Page 361

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CAL
ZO:
SYSTEM
ZO
is
selected
when
the
desired measurement
impedance diers
from the
impedance
of the
line standard.
This
requires
a
knowledge
of
the
exact
value
of
the
Z
0
of
the line
. The
system reference
impedance is
set
using
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SET ZO
under the
calibration menu.
The
actual
impedance
of
the line
is set
by entering
the real
part of
the line
impedance
as
the
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OFFSET
ZO
within
the
dene
standard
menu.
For
example,
if the
line was
known to
have
a
characteristic
impedance
of
51

(
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OFFSET
ZO
=
51
),
it
could still
be used
to calibrate
for
a
50

measurement
(
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SET
ZO
=
50
).
After a
calibration, all
measurements would
be
referenced
to
50
,
instead
of
51
.
When
the
line
standard
is
remeasured, the
center of
the
Smith
chart
is
at
the
current
value
of
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SET
ZO
(in
this
case
,
50
).
Since
only
one
value
of
oset
Z
0
can
be
selected
for the
line standard,
the value
of
Z
0
should
be
a
constant value
over
the
frequency range
of interest
in order
to
be
meaningful.
The
location
of the
reference plane
is determined
by the
selection
of
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SET
REF:
THRU and
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SET
REF: REFLECT
.By
default, the
reference
plane
is
set
with
the
thru
standard
which
must
have
a
known
insertion
phase
or
electrical length.
If a
non-zero length
thru is
specied
to
have
zero
delay
,
the
reference
plane
will
be established
in the
middle
of
the
thru.
The
reect
standard
may
be
used
to
set
the
reference
plane
instead
of
the
thru
provided
the phase
response
(oset
delay
,
reactance
values
and
standard
type)
of
the
reect
standard
is
known
and
is
specied
in
the
calibration
kit
denition.
Note
Dispersion
Eects
Dispersion
occurs
when
a
transmission medium
exhibits a
variable
propagation
or
phase
velocity
as
a function
of frequency
.
The
result
of
dispersion
is
a
non-linear
phase
shift
versus
frequency
,
which
leads
to
a
group
delay
which
is
not
constant.
F
ortunately
,
the
TRL
calibration
technique
accounts
for
dispersive
eects
of
the
test
xture
up
to
the
calibration
plane
,
provided
that:
1. The
thru
(zero
or
non-zero
length)
is
dened
as
having
zero
electrical
length
and
is
used
to
set
the
reference
plane
(
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SET
REF:
THRU
).
2.
The
transmission
lines
used
as
calibration
standards
have
identical
dispersion
characteristics
(i.e
.,
identical
height,
width
and
relative
dielectric
constant).
When
a
non-zero
length
thru
is
used
to
set
the
reference
plane
,
it
should
be
dened
as
having
zero
length
in
the
TRL
standards
denition,
even
though it
has
physical
length.
The
actual
electrical
length
of
the
thru
standard
must then
be
subtracted
from
the
actual
electrical
length
of
each
line
standard
in the
TRL
calibration
kit
denition.
The
device
must
then
be
mounted
between two
short
lengths
of
transmission
line
so
that
each
length
is exactly
one-half
of
the
length
of the non-zero length thru standard. In this conguration, the measurement
will be properly calibrated up to the point of the device
.
6-94 Application and Operation Concepts