HP 54645A, User'S And Service Manual

Page: 93 / 301

Between the 500-
µ
s/div picture (figure 4-1), and the 2-
µ
s/div picture
(figure 4-2), whether or not the pulse or some part of the pulse is displayed is
simply a statistical problem. For every new sweep speed, a new ratio of
displayed-versus-sample points is necessary to provide the “best”
representative picture. Thus, you may see part of the pulse at one sweep
speed, and it may disappear at the next sweep speed.
Whether the scope is stopped or running, you see more detail as you zoom in.
As you zoom out, you see less detail. To keep from losing detail as you zoom
out, switch to peak detect.
Peak Detect
In peak detect display mode, it’s as though the brightness knob is set to
infinity. Any noise, peak, or signal wider than 5 ns will be displayed,
regardless of the Time/Div setting. In Normal display mode, at faster than
2
µ
s/div, you would see a 5-ns peak, so Peak Detect has no effect at faster
than 2
µ
s/div. Note that the
Pk
status indicator is not highlighted in this case.
Consider our previous example, with the scope now in peak detect display
mode. The narrow pulse would always be displayed because the narrow pulse
samples would be considered the maximum value for the given set of
samples. Also note that as we zoom out or in, the narrow pulse would always
be displayed.
Traditionally, in peak detect mode, the only data displayed on the screen has
been the peaks. This can be a disadvantage because all noise or peaks wider
than the minimum peak detect time are always displayed on the scope
screen. This tends to make the signal look noisier than it really is because
emphasis is given to the peaks.
MegaZoom Concepts
Display Modes
4-10