C–19
FFT Glossary
Frequency Resolution In a simple sense, the frequency resolution is equal to the bin
width ∆f. That is, if the input signal changes its frequency by ∆f,
the corresponding spectrum peak will be displaced by ∆f. For
smaller changes of frequency, only the shape of the peak will
change.
However, the effective frequency resolution (i.e. the ability to
resolve two signals whose frequencies are almost the same) is
further limited by the use of window functions. The ENBW value
of all windows other than the rectangular is greater than ∆f and
the bin width. The table on page C–17 lists the ENBW values for
the implemented windows.
Leakage In the power spectrum of a sine wave with an integral number of
periods in the (rectangular) time window (i.e. the source
frequency equals one of the bin frequencies), the spectrum
contains a sharp component whose value accurately reflects the
source waveform's amplitude. For intermediate input
frequencies this spectral component has a lower and broader
peak.
The broadening of the base of the peak, stretching out into
many neighboring bins is termed leakage. It is due to the
relatively high side lobes of the filter associated with each
frequency bin.
The filter side lobes and the resulting leakage are reduced when
one of the available window functions is applied. The best
reduction is provided by the Blackman–Harris and Flat Top
windows. However, this reduction is offset by a broadening of
the main lobe of the filter.
Number of Points FFT is computed over the number of points (Transform Size)
whose upper bounds are the source number of points, and by
the maximum number of points selected in the menu. FFT
generates spectra of N/2 output points.
Nyquist Frequency The Nyquist frequency is equal to one half of the effective
sampling frequency (after the decimation): ∆f × N/2.
Picket Fence Effect If a sine wave has a whole number of periods in the time domain
record, the power spectrum obtained with a rectangular window
will have a sharp peak, corresponding exactly to the frequency
and amplitude of the sine wave. Otherwise the spectrum peak
with a rectangular window will be lower and broader.
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