HP 8753D - Page 355

Source
match and
load match
A
TRL calibration
assumes
a
perfectly
balanced
test
set
architecture
as
shown
by
the
term
which
represents both
the
forward
source
match
(E
SF
)
and reverse
load
match
(E
LR
)
and by
the
"
22
term
which represents
both
the
reverse
source
match
(E
SR
)
and forward
load
match
(E
LF
).
However
,in
any
switching
test
set,
the
source
and
load
match
terms
are
not
equal
because the
transfer
switch presents
a dierent
terminating
impedance
as
it
is
changed
between
port
1
and
port
2.
Because
the
standard
HP
8753D
network
analyzer
is
based
on
a
three-sampler
receiver
architecture
,
it
is
not
possible
to
dierentiate
the
source
match
from
the
load
match
terms
. The
terminating
impedance
of
the
switch
is
assumed
to
be
the
same
in
either
direction.
Therefore
,
the
test
port
mismatch
cannot
be
fully
corrected.
An
assumption
is
made
that:
forward
source
match
(E
SF
)
=
reverse
load
match
(E
LR
)
=
"
11
reverse
source
match
(E
SR
)
=
forward
load
match
(E
LF
)
=
"
22
F
or
a
xture
,
TRL*
can
eliminate
the
eects
of
the
xture's loss
and length,
but does
not
completely
remove
the
eects
due
to
the
mismatch
of
the
xture.
Note
Because
the
TRL
technique
relies
on
the
characteristic
impedance
of
transmission
lines
,
the
mathematically
equivalent
method
LRM
(for
line-reect-match)
may
be
substituted
for
TRL.
Since
a
well
matched
termination
is
,
in
essence,
an innitely
long transmission
line,
it
is
well
suited
for
low
(RF)
frequency
calibrations
.
Achieving
along
line
standard
for
low
frequencies
is
often
times
physically
impossible
.
Improving
Raw
Source
Match
and
Load
Match
F
or
TRL*/LRM*
Calibration
A
technique
that
can
be
used
to
improve
the
raw
test
port
mismatch
is
to
add
high
quality
xed
attenuators
.
The
eective
match
of
the
system
is
improved
because
the
xed
attenuators
usually
have
a
return
loss
that
is
better
than
that
of
the
network
analyzer
.
A
dditionally
,the
attenuators
provide
some
isolation
of
reected
signals
.
The
attenuators
also
help
to
minimize
the
dierence
between
the
port
source
match
and
load
match,
making
the
error
terms
more
equivalent.
With
the
attenuators
in
place
,
the
eective
port
match of
the system
is improved
so
that
the
mismatch
of
the
xture
transition
itself
dominates
the
measurement
errors after
a calibration.
6-88 Application and Operation Concepts