Agilent Technologies InfiniiVision 7000B Series Guide de l'utilisateur télécharger pdf (Page 237)

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Measurements and Math Functions 6

InfiniiVision Oscilloscopes User’s Guide 237

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.

LIASING

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,

rectangular, and Blackman-Harris. For more information on leakage, see Agilent

Application Note 243, “The Fundamentals of Signal Analysis” at

http://cp.literature.agilent.com/litweb/pdf/5952-8898E.pdf.

True frequency component

Aliased frequency component

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Agilent Technologies InfiniiVision 7000B Series Guide de l'utilisateur Page 237

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