HP 54645A User's And Service Manual page 94

Peak Detection Prevents Aliasing
Aliasing produces misleading information. The peak detection circuitry in the
HP 54645A/D helps to prevent aliasing by identifying additional transitions that
occur between logic samples. Because of the high sample rate of the
HP 54645A/D MegaZoom technology, you should not see aliasing.
Consider the following example.
The following picture shows a simple square wave with the scope in peak
detect mode at 1 s/div.
Figure 4-3
Simple Square Wave in Peak Detect Mode on HP 54600
MegaZoom Concepts
Display Modes
4-11