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Acquisition
Operating Basics
1st Acquisition Cycle
3rd Acquisition Cycle
nth Acquisition Cycle
2nd Acquisition Cycle
Record Points
Figure 2-7: Equivalent-Time Sampling
The oscilloscope takes a few samples with each trigger event and eventually
constructs a waveform record using the samples from multiple acquisitions.
That feature lets you accurately acquire signals with frequencies much higher
than the digitizing oscilloscope real-time bandwidth.
The digitizing oscilloscope uses a type of equivalent-time sampling called
random equivalent-time sampling
. Although the samples are taken sequential-
ly in time, they are random with respect to the trigger. That is because the
oscilloscope sample clock runs asynchronously with respect to the input
signal and the signal trigger. The oscilloscope takes samples independent of
the trigger position and displays them based on the time difference between
the sample and the trigger.
The sampling speeds and the number of channels you choose affect the
mode the digitizing oscilloscope uses to sample waveforms. Basically, if the
time base is 200 ns or slower, the digitizing oscilloscope uses real-time sam-
pling for creating waveform records when Fit to Screen is off.
When the time base is faster than 50 ns, the digitizing oscilloscope creates
waveform records using equivalent-time sampling or interpolation. For speeds
between 200 ns and 20 ns, the digitizing oscilloscope creates waveform
records differently depending on the number of input channels and type of
oscilloscope you are using (see Table 2-1).
Selecting Sampling
Mode