
Time
Domain
RCS
Measurements
RCS
W
aveforms
The
HP
8530A
can
provide
a
number
of
dierent
Time
Domain
waveform
shapes
as
the
test
stimulus
.
Three
built-in
waveforms
oer
trade-os
between
Time
Domain
impulse
width
and
sidelobe
levels
.
The
feature
which
provides
dierent
Time
Domain
waveforms
is
called
\windowing."
Windowing
describes
the
technique
of
modifying
the
Frequency
Domain
data
prior
to
its
conversion
to
Time
Domain
data.
The
Windowing
feature
was
provided
to
reduce
ringing
in
the
Time
Domain
response
(the
impulse
\sidelobes"),
or
to
allow
users
to
modify
the
impulse
waveform
to
meet
their
own
needs
.
Sidelobes
are
created
by
the
abrupt
transitions
in
the
Frequency
Domain
measurement
at
the
start
and
stop
frequencies
.
The
\MINIMUM"
window
provides
the
narrowest
Time
Domain
impulse
but
causes
relatively
high
(013
dB)
sidelobes
.
The
resulting
Time
Domain
impulse
is
useful
in
resolving
closely
spaced
scatterers
of
similar
amplitude
,
but
it
is
ineective
in
resolving
target
scatterers
of
widely
dierent
amplitudes
.
The
\NORMAL"
window
provides
a
Time
Domain
impulse
with
a
good
compromise
between
impulse
width
and
sidelobe
level
(044
dB).
The
NORMAL
window
is
the
default
(factory
preset)
selection,
and
it
is
recommended
for
most
RCS
measurements
.
The
\MAXIMUM"
window
provides
a
Time
Domain
impulse
with
no
detectable
sidelobes
,
but
with
a
signicant
increase
in
impulse
width.
T
able
13-1.
Approximate
Impulse
Width
F
ormulas
for
Dierent
Window
Types
(Time
Domain
W
aveforms).
Window
Type
Impulse
Width
F
ormula
Impulse
Sidelobe
Level
Minimum
1.20
4
Frequency
Span
2
1.0
013
dB
Normal
1.20
4
Frequency
Span
2
1.6
044
dB
Maximum
1.20
4
Frequency
Span
2
2.4
090
dB
User-Dened
W
aveforms
F
or
computer
controlled
measurement
systems
,
it
is
possible
to
program
the
HP
8530A
with
a
user-dened
windowing
function
(for
example
,
with
Hamming
or
Hanning
windows).
T
o
do
this
,
load
the
Frequency
Domain
coecients
(for
your
special
window)
into
the
HP
8530A
\Delay
T
able
."
There
is
one
delay
table
for
each
channel.
Initially
,
this
table
must
be
supplied
over
HP-IB
from
a
controller
.
The
delay
table
consists
of
a
complex
(real
and
imaginary)
data
entry
for
each
point
in
the
data
trace
.
Each
entry
can
be
thought
of
as
a
complex
scaling
factor
,
which
is
multiplied
with
the
measured
data
just
after
error
correction
and
before
Time
Domain.
Because
the
operation
takes
place
before
Time
Domain
processing,
the
delay
table
can
be
used
to
create
user-dened
Time
Domain
waveforms
.
With
T
able
Delay
and
the
MINIMUM
window
selected,
the
HP
8530A
will
display
the
Time
Domain
response
of
the
RCS
target
according
to
the
user-supplied
waveform.
Refer
to
\T
able
Delay"
in
the
Keyword
Dictionary
for
more
information.
13-6
Introduction
to
Time
Domain
RCS
and
Antenna
Measurements
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