3-27
Making Time Domain Measurements
Windowing
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 = π/frequency span and t = time (see Figure 3-22). This has two
effects 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 effects of the finite impulse width
cannot be improved without increasing the frequency span of the measurement (see
Table 3-3).
Figure 3-22 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 effects of sidelobes can be improved by windowing (see Table 3-3).
Windowing improves the dynamic range of a time domain measurement by filtering 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.
To select a window, press . A menu is
presented that allows the selection of three window types, see Table 3-3.
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