Agilent Technologies 4395A - Page 418

Calibration
for
Network
Measurement
plane
.The
error contributed
by
directivity
is
independent
of
the
characteristics
of
the
test
device
and it
usually
produces
the
major
ambiguity
in
measurements
of
low
reection
devices
.
Source
Match
Source
match is
dened as
the vector
sum of
signals appearing
at the
analyzer receiver
input
due
to
the
impedance
mismatch
at the
test device
looking back
into the
source
.
Source
match
is
degraded
by
adapters
and
extra cables
.A
non-perfect source
match leads
to
mismatch
uncertainties
that
aect
both
transmission
and reection
measurements.
Source match
is most
often
given in
terms of
return loss
in dB
(therefore
,
the
larger
the
number
,
the
smaller
the
error
.
In
a reection
measurement, the
source match
error signal
is
caused
by
some
of
the
reected
signal
from
the
DUT being
reected from
the source
back
toward
the
DUT
and
re-reected
from
the
DUT
(Figure
A-24
).
In
a transmission
measurement, the
source
match
error
signal
is
caused
by
reection
from
the
test device
that is
re-reected
from
the
source
.
Figure
A-24.
Source Match
The error contributed by source match is a mismatch error caused by the relationship
between
the actual input impedance of the test device and the equivalent match of the source
.Itisa
factor in both transmission and reection measurements
. Mismatch uncertainty is particularly
a problem in measurements where there is a large impedance mismatch at the measurement
plane
.
A-42 Basic Measurement Theory