Tektronix, Inc. P6240, User Manual

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Scaling and Positioning Waveforms
TLS 216 User Manual
2–25
(as opposed to saved reference) waveforms use the same time base, the logic
scope only displays one value for all the active channels.
When aliasing happens, you see a waveform with a frequency lower than the
actual waveform being input or a waveform that is not stable even though the
light next to TRIG’D is lit. Aliasing occurs because the logic scope cannot
sample the signal fast enough to construct an accurate waveform record
(Figure 2-7).
Actual High-Frequency Waveform
Apparent Low-Frequency
Waveform Due to Aliasing
Sampled Points
Figure 2-7: Aliasing
One simple way to check for aliasing is to slowly change the horizontal scale
(time per division setting). If the shape of the displayed waveform changes
drastically, you may have aliasing.
In order to represent a signal accurately and avoid aliasing, you must sample the
signal 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.
There are various ways to prevent aliasing. Try adjusting the horizontal scale, or
simply press the AUTOSET button. You can also counteract some aliasing by
changing the acquisition mode in the Acquisition menu. For example, if you are
using the sample mode and suspect aliasing, try changing to envelope mode.
Since the envelope mode searches for multiple acquisitions with the highest and
lowest values, it can detect faster signal components over time.
You can set a main time base and a delayed time base. Each time base has its
own trigger. There are two types of delayed time base acquisitions, with each
based on its triggering relationship to the main time base. These are delayed runs
after main and delay triggerable (after time, events, or both) acquisitions.
The delayed time base is useful in catching events that follow other events. See
Triggering on page 2–13 for more information on the delayed trigger.
Aliasing
Delayed Time Base