Specifications

Normal Mode
Normal mode is the display mode that you will use most of the time. For the
majority of use models and signals, normal mode yields the best scope
picture of the waveform.
In the case of the HP 54645A/D, the analog channels can have up to 1 million
samples behind the 4,000-point scope display. Some compression of samples
to display points is obviously necessary.
Normal mode follows the model of the dithered system described earlier. In
this case, random sample decimation is used to determine which points will
be displayed. At many sweep speeds, the HP 54645A/D 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, through it has been captured, because of
the sample compression. Consider the following example.
In this example, we are attempting to capture a pulse with a width of ~50 ns,
and the scope is sampling at 200 MSa/s (5 ns/sample). We should get about
10 samples on the pulse of interest. We have captured 1,000,000 sample
points and are displaying a 4,000-point display record. This means that the
dithering scheme is required to pick 1 out of 250 points to display
(1,000,000 / 4,000). In the following figure, note that the narrow pulse is not
displayed.
MegaZoom Concepts
Display Modes
4-8