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STMaT-5
“a
STOP
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n.
(a) Short Circuit
(b)
Short Circuit at
the End of a 3 dB Pad
pg5194_c
Windowing
Figure 6-70. Masking Example
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
stimuh~s
to have a
sin(kt)/kt
shape, where
k = r/frequency span and t
=
time (see Figure 6-71). This has two effects that limit the
usefulness of the time domain measurement:
n
Finite impulse width (or rise time).
F’inite
impulse width limits the ability to resolve
between two closely spaced responses. The effects of the
Gnite
impulse width cannot be
improved without increasing the frequency span of the measurement (see
‘IMAe
6-12).
4
, WINDOWING
SIDELOBES
pg665d
Figure 6-71. Impulse Width, Sidelobes, and Windowing
n
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 effects of
sidelobes can be improved by windowing (see
‘J%ble
6-12).
Windowing improves the dynamic range of a time domain measurement by
altering
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 different in magnitude. The
sidelobe
reduction is achieved, however, at the expense of
increased impulse width. The effect of windowing on the step stimulus (low pass mode only) is
a reduction of overshoot and ringing at the expense of increased rise time.
B-136 Application and Operation Concepts