Agilent Technologies 54622A User Manual page 117

Normal Mode
For the majority of use models and signals, Normal acquisition mode yields the
best oscilloscope picture of the waveform.
The analog channels can have up to 2 million samples behind the 1,000-point
oscilloscope display. Some compression of samples to display points is obviously
necessary due to the high plot rate (up to 25 million vectors/second). The
compression is much less that previous oscilloscopes. This means the signal
will look "right" at many more operating points.
Peak Detect
Normal mode is a dithered system in which random sample decimation is used
to determine which points will be displayed. At many sweep speeds, the
oscilloscope captures many more points than can be displayed—in these cases,
some points will not be displayed. This can present a problem if you are
interested in capturing a narrow event relative to the time window of interest.
Keep in mind that this narrow pulse may not be displayed, though it has been
captured, because of the sample compression.
For example, if we are attempting to capture a pulse with a width of ~50 ns, and
the oscilloscope is sampling at 200 MSa/s (5 ns/sample), we should get about
10 samples on the pulse of interest. We have captured 2,000,000 sample points
and are displaying a 1,000-point display record. This means that the dithering
scheme picks 1 out of 2000 points to display (2,000,000/1,000), and the pulse
is probably not among them.
Peak detect displays any signal pulse wider than 5 ns, allowing you to find
glitches and narrow spikes regardless of sweep speed. Because of MegaZoom
memory, peak detect is only necessary at sweep speeds of 1 ms/div and slower.
At faster sweep speeds, Normal mode can capture the narrow peaks.
At sweep speeds slower than 1 ms/div, the oscilloscope can acquire more
samples than can fit into MegaZoom memory. The oscilloscope selects samples
to store such that no local minimum or maximum wider than 5 ns is missed. In
Peak Det mode, narrow glitches and sharp edges are displayed more brightly
than when in Normal mode, making them easier to see.
All points displayed after Single or Stop
After a Single acquisition or when the oscilloscope has been stopped, all acquired
points are displayed.
MegaZoom Concepts and Oscilloscope Operation
Acquisition Modes
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