User manual

FFT Analyzer
4-82
AM700 Audio Measurement Set User Manual
Measurement Bandwidth
The FFT Analyzer has two measurement bandwidths. The high resolution mode
provides a useable bandwidth of DC to 20 kHz, and the high bandwidth mode
provides a useable bandwidth of DC to 80 kHz. Displayed data can be acquired over
a changing span of 400 Hz to 80 kHz.
Windowing
Weighting functions are applied to the sampled window to reduce the effects of
out-of-window spectral components. A choice of Hann, Kaiser-Bessel, BHR
(Blackman-Harris), Flat-top, Uniform (no weighting), Saramäki Rajan, and a user-
defined weighting function can be selected for use with the FFT Analyzer using the
Window Selection menus shown in Figure 4-37. The effects of the different
windows are shown in Figure 4-38.
Uniform. The Uniform window provides no weighting of the data. A signal will
appear as narrow as a single frequency bin if it has a frequency exactly equal to a
frequency bin. If a frequency lies between frequency bins its presence will be seen
across the spectrum. The amplitude variation between the two cases is quite large.
The Uniform window can be used to look for transients that do not fill the entire
time record. A particular use of this window for multitone measurement occurs
when the multitone frequencies lie exactly on the bin frequencies.
Hann. The Hann window is most often used as a good compromise between
selectivity and amplitude accuracy. The roll off produced by the weighting function
is not very steep; therefore, performance can be reduced when viewing the fft of a
signal composed of close frequency components that are very different in amplitude.
Other windows provide either more resolution or better amplitude accuracy and can
be selected for a particular application.
Flat-top. Amplitude accuracy is improved using the flat-top window. Amplitude
variations of signal components that are between frequency bins vary roll off only a
small amount (about 0.02 dB). While the selectivity is reduced by the width of the
flat-top, the roll off is very steep and adjacent frequency components can be
resolved. While the signals may appear wide, they do not leak out of the bins.