Agilent Technologies 4395A - Page 419

Calibration
for
Network
Measurement
Load
Match
Load
match
error
results
from
an
imperfect
match
at
the
output of
the test
device.
It is
caused
by
impedance
mismatches
between
the
test
device output
port and
port 2
of the
measurement
system.
As
illustrated
in
Figure
A
-25,
some of
the transmitted
signal is
reected from
port
2
back
to
the
test
device
.A
portion of
this wave
can be
re-reected to
port 2,
or part
can
be
transmitted
through
the
device
in
the
reverse
direction
to
appear at
port 1.
If the
DUT
has
low
insertion
loss
(for
example
a
transmission
line),
the
signal reected
from port
2and
re-reected
from
the
source
causes
a
signicant error
because the
DUT does
not attenuate
the
signal
signicantly
on
each
reection.
Load
match is
usually given
in terms
of return
loss
in
dB
(therefore
,
the
larger
the
number
, the
smaller the
error).
Figure
A
-25.
Load
Match
The
error
contributed
by
load match
depends on
the relationship
between
the
actual
output
impedance
of
the
test
device and
the eective
match of
the
return
port
(port
2).
It
is
a
factor
in
all
transmission
measurements
and in
reection measurements
of two-port
devices.
Load
match and
source match
are
usually
ignored
when
the
test
device
insertion
loss
is
greater
than
about 6
dB.
This happens
because the
error
signal
is
greatly
attenuated
each
time
it
passes
through the DUT
. However
, load match eects produce major
transmission measurement errors
for a test device with a highly reective output port.
Isolation (Crosstalk)
Leakage of energy between analyzer signal paths contributes to error in a transmission
measurement much as directivity does in a reection measurement. Isolation is the vector sum
of signals appearing at the analyzer receivers due to crosstalk between the reference and test
signal paths, including signal leakage within the test set and in both the RF and IF sections of
the receiver.
The error contributed by isolation depends on the characteristics of the DUT. Isolation is a
factor in high-loss transmission measurements. However, analyzer system isolation is more than
sucient for most measurements, and correction for it may be unnecessary.For measuring
devices with high dynamic range, accuracy enhancement can provide improvements in isolation
that are limited only by the noise oor.
Basic Measurement Theory A-43