Agilent Technologies 4395A - Page 391

Network
Measurement
Basics
Smith
Chart
A
Smith
chart
is
used
in
reection
measurements
to
provide
a readout
of the
data in
terms of
impedance
.
The
intersecting
lines
on
a Smith
chart represent
constant resistance
and constant
reactance
values
,
normalized
to
the
characteristic impedance
,Z
0
,
of
the
system.
Reactance
values
in
the
upper
half
of
the Smith
chart circle
are positive
(inductive) reactance
,and
in the
lower
half
of
the
circle
are
negative
(capacitive)
reactance
.
P
olar Chart
Each
point on
the polar
format corresponds
to a
particular
value
of
both
magnitude
and
phase
.
Quantities
are read
vectorally: the
magnitude at
any point
is
determined
by
its
displacement
from
the center
(which has
zero
value),
and
the
phase
by
the
angle
counterclockwise
from
the
positive x-axis
. Magnitude
is
scaled
in
a
linear
fashion,
with
the
value
of
the
outer
circle
usually
set to
a ratio
value
of
1.
Because
there
is
no
frequency
axis
,
frequency
information
is
read
from the
markers.
Electrical
Delay
The
electrical
delay
function
simulates
a
variable
length
loss-free
transmission
line
that
can
be
added to
or removed
from a
receiver
input
to
compensate
for
interconnecting
cables
,
etc
.
This
function is
similar to
the mechanical
or analog
\line
stretchers"
of
conventional
network
analyzers
.
Delay
is annotated
in units
of
time
with
secondary
labeling
in
electrical
length,
associated
with
equivalent
length
of
the
transmission
line
if
a value
for
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VELOCITY
FACTOR
(see
below)
is
specied.
T
o
obtain the
characteristics
of
the
DUT
itself
free
of
the
inuence
of
interconnecting
cables
,
use
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ELECTRICAL
DELAY
under
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ELECTRICAL
DELAY
MENU
,
and
enter
the
following
setting:
Electrical
delay
1
t
=
1

F
2
360
[
sec
]
where:
F
(Hz)
Measuring
frequency
1

(degree)
Dierence
between
measuring
frequency
without
cables
and
that
with
the
cables
connected
In
this case
,
the
4395A
displays
the
electrical
length
of
the
interconnecting
cable
to
compensate
for
, along
with
the
value
of
electrical
delay:
1
l
=
v
o
2
1 t
[
m
]
where:
vo (=2.997925E8) (m/sec)
light velocity in vacuum
If the average relative permittivity (

r
) of the DUT is known over the frequency span, the
length calculation can be adjusted to reect the actual length of the DUT more closely
. This
can be done by entering the relative velocity factor for the DUT using
NNNNNNNNNNNNNNNNNNNN
NNNNNNNNNNNNNNNNNNNNNNNNN
NN
VELOCITY FACTOR
under the
4
Cal
5 key:
1
p

r
assuming a relative permeability of 1.
Basic Measurement Theory A-15