Tektronix CSA7404B, User Manual

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Acquiring Waveforms
CSA7000B Series & TDS7000B Series Instruments User Manual
3- 29
Preventing Aliasing. Under certain conditions, a waveform may be aliased on
screen. Read the following description about aliasing and the suggestions for
preventing it.
When a waveform aliases, it appears on screen with a frequency lower than the
actual waveform being input or it appears unstable even though the TRIG’D
light is lighted. Aliasing occurs because the instrument is not sampling the signal
fast enough to construct an accurate waveform record. (See Figure 3--7.)
Actual high-frequency
waveform
Apparent low-frequency
waveform due to aliasing
Sampled points
Figure 3- 7: Aliasing
Methods to Check and Eliminate Aliasing.
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 to avoid aliasing. The
following tips may help you eliminate aliasing on a signal:
H
Turn on Fast Acquisition mode to maximize the waveform capture rate.
H
Turn on Waveform Database mode to capture more data.
H
Try adjusting the horizontal scale for proper waveform display.
H
Try pushing the AUTOSET button.
H
Try switching the acquisition to Envelope mode. Envelope searches for
samples with the highest and lowest values over multiple acquisitions and
can detect faster signal components over time.
H
Turn on PeakDetect acquisition mode. If the waveform becomes an
envelope, aliasing was occurring.
Use the procedure that follows to set the data acquisition mode and specify
To Set Acquisition Modes