LeCroy DDA-3000A, Operator'S Manual

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WM-OM-E Rev I
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.
Window Type Applications and Limitations
Rectangular These are normally used when the signal is transient (completely contained in
the time-domain window) or known to have a fundamental frequency
component that is an integer multiple of the fundamental frequency of the
window. Signals other than these types will show varying amounts of spectral
leakage and scallop loss, which can be corrected by selecting another type of
window.
Hanning (Von
Hann)
These reduce leakage and improve amplitude accuracy. However, frequency
resolution is also reduced.
Hamming These reduce leakage and improve amplitude accuracy. However, frequency
resolution is also reduced.
Flat Top This window provides excellent amplitude accuracy with moderate reduction of
leakage, but with reduced frequency resolution.
Blackman-Harris It reduces the leakage to a minimum, but with reduced frequency resolution.