Scaling and Positioning Waveforms
TDS 620A, 640A, & 644A User Manual
Aliasing
When

aliasing

happens, you see a waveform with a frequency lower than the
actual waveform being input or a waveform is not stable even though the light
next to TRIG’D is lit. Aliasing occurs because the oscilloscope cannot sample
the signal fast enough to construct an accurate waveform record (Figure 2-9).
Actual High-Frequency Waveform
Apparent Low-Frequency
Waveform Due to Aliasing
Sampled Points
Figure 2-9: 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, you may want to
change to the peak detect mode. Since the peak detect mode searches for
samples with the highest and lowest values, it can detect faster signal compo-
nents over time.
Delayed Time Base
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. Each
type is 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 displaying events that follow other events.
See
Triggering
on page 2-13 for more information on the delayed trigger.