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Using Features for Advanced Applications
TDS 500C, TDS 600B, & TDS 700C User Manual
3–209
Leakage results when the time domain waveform delivered to the FFT function
contains a non-integer number of waveform cycles. Since there are fractions of
cycles in such records, there are discontinuities at the ends of the record. These
discontinuities cause energy from each discrete frequency to “leak” over on to
adjacent frequencies. The result is amplitude error when measuring those
frequencies.
The rectangular window does not modify the waveform record points; it
generally gives the best frequency resolution because it results in the most
narrow lobe width in the FFT output record. If the time domain records you
measured always had an integer number of cycles, you would only need this
window.
Hamming, Hanning, and Blackman-Harris are all somewhat bell-shaped widows
that taper the waveform record at the record ends. The Hanning and Blackman/
Harris windows taper the data at the end of the record to zero; therefore, they are
generally better choices to eliminate leakage.
Care should be taken when using bell shaped windows to be sure that the most
interesting parts of the signal in the time domain record are positioned in the
center region of the window so that the tapering does not cause severe errors.