Agilent Technologies 4000 X-Series User Manual page 104

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.
Nyquist Frequency and Aliasing in the Frequency Domain
N O T E
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.
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 horizontal time/div setting
for the square wave sets the sample rate and results in a FFT resolution
of 1.91 Hz. The displayed FFT spectrum 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.
104 Agilent InfiniiVision 4000 X-Series Oscilloscopes User's Guide