Windowing; General; Windows Used With This Analyzer - Agilent Technologies 89410A Operator's Manual

Fundamental Measurement Interactions

Windowing

General

A window is a time-domain weighting function applied to the input signal. A
window is a filter used to compensate for the fact that most signals are not periodic
within the input time record. Depending on the window, the analyzer attenuates
the ends of the input time record, to prevent leakage—a smearing of energy across
the frequency spectrum—caused by transforming signals that are not periodic
within the time record.
FFT analyzers usually have several window types available. Each window offers
particular advantages. Because each window type produces different measurement
results (just how different depends on the characteristics of the input signal and
how you trigger on it), you should carefully select a window type appropriate for
the measurement you're trying to make.
Windowing is a concept basic to understanding FFT spectrum analyzers. To learn
more, see Spectrum and Network Measurements by Robert A. Witte. Refer also to
online help for a description of each window type and guidelines on choosing a
window type.

Windows used with this analyzer

This analyzer functions as if the input signal were applied to a parallel bank of
narrow-band filters (the number corresponding to the number of frequency points).
The illustrations on the next page show the frequency-domain response of a single
filter when using uniform, Hanning, gaussian top, or flattop windows.
The left side of each illustration represents the center of each filter. Since the
filters are symmetrical, we've shown only one side of each filter response (the
other side is a mirror image). The horizontal axis is normalized to 1/ T.
Think of each drawing as a template. If you position a sine frequency at the exact
center of the filter, more of a sine wave's energy appears in the center bin. Some of
its energy also appears in other bins. The amount of energy that spills into adjacent
bins depends on the type of window you use. Frequency resolution is determined
by the width of the main lobe. The Hanning window provides better frequency
resolution for a fixed time record length while the Flattop window has good
frequency resolution for a fixed resolution bandwidth.
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