Keysight Technologies InfiniiVision 3000 X Series, User Manual

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Math Waveforms
4
Keysight InfiniiVision 3000 X-Series Oscilloscopes User's Guide 89
The following figure illustrates aliasing. This is the spectrum of a 990 Hz square
wave, which has many harmonics. The sample rate is set to 100 kSa/s, and the
oscilloscope displays the spectrum. The displayed waveform shows the
components of the input signal above the Nyquist frequency to be mirrored
(aliased) on the display and reflected off the right edge.
Because the frequency span goes from
≈
0 to the Nyquist frequency, the best way
to prevent aliasing is to make sure that the frequency span is greater than the
frequencies of significant energy present in the input signal.
FFT Spectral Leakage
The FFT operation assumes that the time record repeats. Unless there is an
integral number of cycles of the sampled waveform in the record, a discontinuity is
created at the end of the record. This is referred to as leakage. In order to minimize
spectral leakage, windows that approach zero smoothly at the beginning and end
of the signal are employed as filters to the FFT. The FFT Menu provides four
windows: Hanning, Flat Top, Rectangular, and Blackman-Harris. For more
Figure 9 Aliasing