Figure 32 Aliasing - Agilent Technologies 7000B Series User Manual

5 Measurements and Math Functions
Aliasing happens when there are frequency components in the signal
higher than half the sample rate. Since the FFT spectrum is limited by
this frequency, any higher components are displayed at a lower (aliased)
frequency.
The following figure illustrates aliasing. This is the spectrum of a 990 Hz
square wave, which has many harmonics. The FFT 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.
Figure 32
Since 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.
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, flattop,
198
Aliasing
True frequency component
Aliased frequency component
InfiniiVision 7000B Series Oscilloscopes User's Guide