Tektronix TDS 500C, TDS 600B, TDS 700C Preventing Aliasing

Acquiring and Displaying Waveforms
TDS 500C, TDS 600B, & TDS 700C User Manual 3–37
NOTE. To quickly select Single Acquisition Sequence without displaying the
Acquire and Stop After menus, press SHIFT FORCE TRIG. Now the RUN/STOP
button operates as just described. (You still must display the Acquire menu and
then the Stop After menu to leave Single Acquisition Sequence operation.)
Press Limit Test Condition Met (side) to acquire waveforms until
waveform data exceeds the limits specified in the limit test. Then acquisition
stops. At that point, you can also specify other actions for the oscilloscope to
take, using the selections available in the Limit Test Setup main menu.
NOTE. For the oscilloscope to stop an acquisition when limit test conditions have
been met, limit testing must be turned ON using the Limit Test Setup main menu.
Setting up limit testing requires several more steps. See Limit Testing on
page 3–183.
Under certain conditions, a waveform may be aliased on screen. Read the
following description about aliasing and the suggestions for preventing it.
About Aliasing. 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 light next to TRIG’D is lighted. Aliasing occurs because the oscilloscope
cannot sample the signal fast enough to construct an accurate waveform record.
(See Figure 3–18.)
Actual High-Frequency Waveform
Apparent Low-frequency
Waveform Due to Aliasing
Sampled Points
Figure 3–18: Aliasing
Preventing Aliasing