74 Agilent InfiniiVision 2000 X-Series Oscilloscopes User's Guide
4 Math Waveforms
span, and oscilloscope vertical bandwidth when making FFT
measurements. The FFT resolution (the quotient of the sampling rate and
the number of FFT points) is displayed directly above the softkeys when
the FFT Menu is displayed.
Aliasing happens when there are frequency components in the signal
higher than half the sample rate. Because 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 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.
Nyquist Frequency and Aliasing in the Frequency Domain
The Nyquist frequency is the highest frequency that any real-time digitizing oscilloscope
can acquire without aliasing. This frequency is half of the sample rate. Frequencies above
the Nyquist frequency will be under sampled, which causes aliasing. The Nyquist
frequency is also called the folding frequency because aliased frequency components fold
back from that frequency when viewing the frequency domain.
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