Agilent Technologies 24A Guia do Utilizador descarregar pdf (Página 207)

3-27

Making Time Domain Measurements

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.

System

TRANSFORM MENU

WINDOW

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Agilent Technologies 24A Guia do Utilizador Página 207