SDA Operator’s Manual
If a signal contains a frequency component above Nyquist, the spectrum will be aliased, meaning
that the frequencies will be folded back and spurious. Spotting aliased frequencies is often
difficult, as the aliases may ride on top of real harmonics. A simple way of checking is to modify
the sample rate and observe whether the frequency distribution changes.
Leakage
FFT assumes that the signal contained within the time grid is replicated endlessly outside the
observation window. Therefore if the signal contains discontinuities at its edges, pseudo-
frequencies will appear in the spectral domain, distorting the real spectrum. When the start and
end phase of the signal differ, the signal frequency falls within two frequency cells, broadening
the spectrum.
The broadening of the base, stretching out in many neighboring bins, is termed leakage. Some
cures for this problem include ensuring an integral number of periods are contained within the
display grid or that no discontinuities appear at the edges. Another is to use a window function to
smooth the edges of the signal.
Choosing a Window
The choice of a spectral window is dictated by the signal’s characteristics. Weighting functions
control the filter response shape, and affect noise bandwidth as well as side lobe levels. Ideally,
the main lobe should be as narrow and flat as possible to effectively discriminate all spectral
components, while all side lobes should be infinitely attenuated. The window type defines the
bandwidth and shape of the equivalent filter to be used in the FFT processing.
In the same way as one would choose a particular camera lens for taking a picture, some
experimenting is generally necessary to determine which window is most suitable. However, the
following general guidelines should help.
Rectangular windows provide the highest frequency resolution and are thus useful for estimating
the type of harmonics present in the signal. Because the rectangular window decays as a (sinx)/x
function in the spectral domain, slight attenuation will be induced. Alternative functions with less
attenuation (Flat Top and Blackman-Harris) provide maximum amplitude at the expense of
frequency resolution; whereas, Hamming and Von Hann are good for general purpose use with
continuous waveforms.
FFT Window Filter Parameters
Window Type Highest Side
Lobe
(dB)
Scallop Loss
(dB)
ENBW
(bins)
Coherent Gain
(dB)
Rectangular -13 3.92 1.0 0.0
von Hann -32 1.42 1.5 -6.02
Hamming -43 1.78 1.37 -5.35
Flat Top -44 0.01 2.96 -11.05
Blackman-Harris -67 1.13 1.71 -7.53
SDA-OM-E Rev H 197
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