Another
reason for
showing this
example is
to
point
out
the
potential
problem
in
calibrating
at
low
frequencies using
TRL.
F
or
example
,
one-quarter
wavelength
is
Length
(
cm
)=
7500
2
V
F
f
c
where:
fc
=
center
frequency
Thus
,at
50 MHz,
Leng
th
(
cm
)
=
7500
50
(
M
H
z
)
=
150
cm
or
1
:
5
m
Such
a
line
standard
would
not
only
be
dicult
to
fabricate
,
but
its
long
term stability
and
usability
would
be
questionable
as
well.
Thus
at
lower
frequencies
and/or
very
broad
band
measurements
,
fabrication
of a
\match" or
termination
may
be
deemed
more
practical.
Since
a
termination
is
,
in essence
,an
innitely
long
transmission
line
,
it
ts
the
TRL
model
mathematically
,
and is
sometimes referred
to as
a
\TRM"
calibration.
The
TRM
calibration
technique
is
related
to
TRL
with
the
dierence
being
that
it
bases
the
characteristic
impedance
of
the
measurement
on
a
matched
Z
O
termination
instead
of
a
transmission
line
for
the
third
measurement
standard.
Like
the
TRL thru
standard, the
TRM
THRU
standard
can
either
be
of
zero
length
or
non-zero length.
The same
rules for
thru and
reect
standards
used
for
TRL
apply
for
TRM.
TRM
has
no
inherent frequency
coverage limitations
which
makes
it
more
convenient
in
some
measurement
situations
.
A
dditionally
, because
TRL
requires
a
dierent
physical
length
for
the
thru
and
the
line
standards
,its
use
becomes
impractical
for
xtures
with
contacts
that
are
at
a
xed
physical
distance
from
each
other
.
F
or
information
on
how
to
modify
calibration
constants
for
TRL*/LRM*,
and
how
to
perform
a
TRL
or
TRM
calibration,
refer
to
Chapter
5,
\Optimizing
Measurement
Results
."
TRL
Options
The
N
N
N
N
N
N
N
NN
NN
NN
NN
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
NN
NN
NN
NN
N
N
N
N
TRL/LRM
OPTION
softkey
provides access
to the
TRL/LRM options
menu. There
are
two
selections under
this
menu:
NN
NN
NN
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
CAL ZO:
(calibration Z
0
)
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
NN
NN
NN
NN
SET
REF:
(set
reference)
The
characteristic
impedance used
during
the
calibration
can
be
referenced
to
either
the
line
(or match) standard (
NN
NNNNNNNNNNNNNNNNNNNNNNNNN
NNNNNNNNNNNNNNNNNNNN
CAL ZO: LINE ZO
) or to the system (
NN
NNNNNNNNNNNNNNNNNNNNNNNNN
NNNNNNNNNNNNNNNNNNNNNNNNN
N
CAL ZO: SYSTEM ZO
). The analyzer
defaults to a calibration impedance that is equal to the line (or match) standard.
When the
NNNNNNNNNNNNNN
NNNNNNNNNNNNNNNNNNNNNNNNN
NNNNNNNN
CAL ZO: LINE ZO
is selected, the
impedance of the line (or match) standard is
assumed to match the system impedance
exactly (the line standard is reectionless). After
a calibration, all measurements are referenced to the impedance of the line standard. F
or
example, when the line standard is remeasured, the response will appear at the center of
the
Smith chart. When
NN
NNNNNNNNNNNNNNNNNNNNNNNNN
NNNNNNNNNNNNNNNNNNNN
CAL ZO: LINE ZO
is selected, the values entered for
NN
NNNNNNNNNNNNNNNNNN
SET ZO
(under CAL
menu) and
NNNNNNNNNNNNNNNNNNNNNNNNNNNNN
OFFSET ZO
(within the dene standard menu) are ignored.
Application and Operation Concepts 6-93
To Next Page
Loading...
