HP 8753D - Page 395

Figure
6-69.
Masking
Example
Windowing
The
analyzer
provides
a
windowing
feature
that
makes
time
domain
measurements
more
useful
for
isolating
and
identifying
individual
responses
.
Windowing
is
needed
because
of
the
abrupt
transitions
in
a
frequency
domain
measurement
at
the
start
and
stop
frequencies
.
The
band
limiting
of
a
frequency
domain
response
causes
overshoot
and
ringing
in
the
time
domain
response
,
and
causes
a
non-windowed
impulse
stimulus
to
have
a
sin(kt)/kt
shape
,
where
k
=

/frequency
span
and
t
=
time
(see
Figure
6-70
).
This
has
two eects
that
limit
the
usefulness
of
the
time
domain
measurement:
Finite
impulse
width
(or
rise
time).
Finite
impulse
width
limits
the
ability
to
resolve
between
two
closely
spaced
responses
.
The
eects
of the
nite impulse
width
cannot
be
improved
without
increasing
the
frequency
span
of
the
measurement (see
Table
6-11
).
Figure 6-70. Impulse Width, Sidelobes
, and Windowing
Sidelobes. The impulse sidelobes limit the dynamic range of the time domain
measurement
by hiding low-level responses within the sidelobes of higher level responses
. The eects
of
sidelobes can be improved by windowing (see Table 6-11).
Windowing improves the dynamic range of a time domain measurement by ltering the
frequency domain data prior to converting it to the time domain, producing an impulse
stimulus that has lower sidelobes. This makes it much easier to see time domain responses that
are very dierent in magnitude. The sidelobe reduction is achieved, however, at the expense of
6-128 Application and Operation Concepts