C–17
FFT Glossary
Glossary
Defines the terms frequently used in FFT spectrum analysis
and relates them to the oscilloscope.
Aliasing If the input signal to a sampling acquisition system contains
components whose frequency is greater than the Nyquist
frequency (half the sampling frequency), there will be less than
two samples per signal period. The result is that the contribution
of these components to the sampled waveform is
indistinguishable from that of components below the Nyquist
frequency. This is aliasing.
The timebase and transform-size should be selected so that the
resulting Nyquist frequency is higher than the highest significant
component in the time-domain record.
Coherent Gain The normalized coherent gain of a filter corresponding to each
window function is 1.0 (0 dB) for a rectangular window and less
than 1.0 for other windows. It defines the loss of signal energy
due to the multiplication by the window function. This loss is
compensated in the oscilloscope. This table lists the values for
the implemented windows.
Window Frequency-Domain 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
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