Math setups Aliasing
Aliasing
Problems occur when the instrument acquires a signal containing frequency components that are higher
in frequency than the Nyquist frequency
(see page 173). The frequency components that are above the
Nyquist freq
uency are under sampled and a ppear to "fold back" around the Nyquist frequency, showing as
lower frequency components. These incorrect components are called aliases.
To quickly c
heck for aliasing, slowly decrease the horizontal time scale. If the shape of the displayed
waveform changes drastically or becomes stable, your waveform is probably aliased. You can also check
for aliasing by turning on peak detect acquisition mode. To remove aliasing, modify the record length.
Eliminate aliasing
Use the following methods to eliminate aliases:
Increase the sample rate by adjusting the horizontal scale to a faster time-per-division setting. Since
you increase the Nyquist frequency
(see page 173) as you increase the time per division, the aliased
frequency components should appear at their proper frequency. If the increased number of frequency
components shown on the screen makes it difficult to measure individual components, use MultiView
Zoom to magnify the FFT waveform.
Use a filter on the source signal to limit the bandwidth of the signal to frequencies below that of
the Nyquist frequency. If the frequency components that you want to view are below the built-in
bandwidth settings, set the source channel bandwidth to the appropriate value.
Push the Autoset button.
Switch the acquisition to Envelope mode or Peak Detect mode (see page 193). Envelope mode
searches for samples with the highest and lowest values over multiple acquisitions and c an detect
faster signal components over time. In Peak Detect mode, the acquisition alternates between saving
the highest sample in one acquisition interval and lowest sample in the next acquisition interval.
256 DSA/DPO70000D, MSO/DPO/DSA70000C, DPO7000C, and MSO/DPO5000 Series
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