Agilent Technologies 4395A - Page 426

Calibration
for
Network
Measurement
Figure
A
-28.
Eective
Directivity
E
DF
Source
match
error
. Because
the measurement
system test
port
is
never
exactly
the
characteristic
impedance
(50
or
75 ),
some of
the
reected
signal
is
re-reected
o
the
test
port,
or
other
impedance
transitions
further down
the line
,
and
back
to
the
unknown,
adding
to
the
original
incident
signal
(I).
This
eect
causes
the
magnitude and
phase
of
the
incident
signal
to
vary
as
a
function
of
S
11A
and
frequency
.
Leveling
the
source
to
produce
constant
(I)
reduces
this
error
,
but
because
the
source
cannot
be
exactly
leveled
at
the
test
device
input,
leveling
cannot
eliminate
all
power
variations
.
This
re-reection
eect
and
the
resultant
incident
power
variation
are
caused
by
the
source
match
error
,
E
SF
(
Figure
A
-29
).
Figure
A-29.
Source
Match
E
SF
Frequency
response
error.
Frequency response
(tracking) error
is caused
by
variations
in
magnitude and
phase atness
versus frequency
between the
test and
reference signal
paths
.
These
are
due
mainly
to
imperfectly
matched
receiver
circuits
and
dierences
in
length and
loss between incident and test signal paths
. The vector sum of these variations is the reection
signal path tracking error
,E
RF
(Figure A
-30).
A-50 Basic Measurement Theory