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Your choice of window function will depend on the input source characteristics that you want to
observe and the characteristics of the window function. The window characteristics are shown in
the following table.
Window 3 dB BW
in bins
Scallop
loss
Nearest
side lobe
Zero
phase
reference
Coefficients
Rectangular 0.89 3.96 dB -13 dB 50% 1.0
Hamming 1.3 1.78 dB -43 dB 50% 0.543478,
0.456522
Hanning 1.44 1.42 dB -32 dB 50% 0.5, 0.5
Kaiser–
Bessel
1.72 1.02 dB -69 dB 50% 0.40243,
0.49804,
0.09831, 0.00122
Blackman–
Harris
1.92 0.81 dB -92 dB 50% 0.35875,
0.48829,
0.14128, 0.01168
Gaussian 2.0 0.76 dB -79 dB 50% a = 3.75 (not
cosine series)
Flattop2 3.8 0.0065
dB
-90 dB 50% 0.213348, -
0.206985,
0.139512, -
0.043084,
0.003745
Tek
Exponential
1.42 0.60 dB -67 dB 20% na
3 dB BW in Bins: This is the bandwidth in units of bins, which are the intervals between spectral
output samples when no zero fill is used. The bandwidth is measured between the points on the
lobe that are 3 dB down from the peak of the lobe. The bandwidth in hertz may be computed by
dividing the BW in bins by the gate duration in seconds. This is also referred to as resolution
bandwidth (RBW).
Coherent gain: The gain factor normally associated with different window functions is correctly
scaled into the magnitude spectrum output. Therefore, the magnitudes in the output spectrum do
not change as different windows are selected.
Scallop Loss: This is the magnitude error of the spectral analyzer when the frequency of the
observed signal is exactly half way between two frequency samples of the spectrum when the
interpolation ratio due to zero fill of the FFT is one. When zero fill is in effect, scallop loss is
essentially eliminated because of the interpolation in the frequency domain due to zero fill. If you
work with span settings less than full and you work with larger resolution bandwidth settings, zero
fill is in effect most of the time.
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