Agilent Technologies 54657A, User Manual

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Aliasing When using FFT’s, it is important to be aware of aliasing. This
requires that the operator have some knowledge as to what the
frequency domain should contain, and also consider the effective
sampling rate, frequency span, and oscilloscope vertical bandwidth when
making FFT measurements. Effective sample rate is briefly displayed
when the
±
key is pressed.
Aliasing happens when there are insufficient samples acquired on each cycle
of the input signal to recognize the signal. This occurs whenever the
frequency of the input signal is greater than the Nyquist frequency (sample
frequency divided by 2). When a signal is aliased, the higher frequency
components show up in the FFT spectrum at a lower frequency.
The following figure illustrates aliasing. In waveform A, the sample rate is set
to 200 kSa/s, and the oscilloscope displays the correct spectrum. In
waveform B, the sample rate is reduced by one-half (100 kSa/s), causing the
components of the input signal above the Nyquist frequency to be mirrored
(aliased) on the display.
Aliasing
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 present in the input signal.
Figure 2– 2
Operating the Measurement/Storage Module
FFT Measurement
2–9