224 6000 Series Oscilloscope User’s Guide
5 Making Measurements
vertical bandwidth when making FFT measurements. The FFT
sample rate 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. 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 100kSa/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 31 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 of significant
energy present in the input signal.
True frequency component
Aliased frequency component