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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 eects 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 eects of the nite impulse width cannot be
improved without increasing the frequency span of the measurement (see T able 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 eects of
sidelobes can be improved by windowing (see T able 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 dierent in magnitude . The sidelobe reduction is achieved, however , at the expense of
6-128 Application and Operation Concepts