Acquiring and Displaying Waveforms
TDS 684A, TDS 744A, & TDS 784A User Manual 3–27
Actual High-Frequency Waveform
Apparent Low-frequency
Waveform Due to Aliasing
Sampled Points
Figure 3–16: Aliasing
Methods to Check and Eliminate. To quickly check for aliasing, slowly increase
the horizontal scale (time per division setting). If the shape of the displayed
waveform changes drastically or becomes stable at a faster time base setting,
your waveform was probably aliased.
To avoid aliasing, be sure to sample the input signal at a rate more than twice as
fast as the highest frequency component. For example, a signal with frequency
components of 500 MHz would need to be sampled at a rate faster than
1 Gigasamples/second to represent it accurately and avoid to aliasing. The
following tips may help you eliminate aliasing on a signal:
Try adjusting the horizontal scale.
Try pressing the AUTOSET button.
Try switching the acquisition mode (in the acquisition menu) to Envelope
(all three TDS models) or Peak Detect (TDS 700A models only). Envelope
searches for samples with the highest and lowest values over multiple
acquisitions; Peak Detect mode does the same but in a single acquisition.
Either can detect faster signal components over time.
Try pressing the InstaVu acquisition button. (TDS 700A models only).
InstaVu mode results in waveform displays similar to those obtained using
analog oscilloscope, due to its fast waveform capture rate.
Customizing the Display
The TDS Oscilloscope can display waveform records and other display elements
in different ways. This subsection describes how to adjust the oscilloscope
display style, intensity level, graticule, and format.