HP 54710A User's Reference Manual page 223

Measurements
Time-interval measurements
Rise Time Response
If a step with a Gaussian shape is applied to the input of the HP 54720, the
measured 10 percent to 90 percent rise time is approximately equal to the
quadrature sum of the rise time of the step and the rise time of the
oscilloscope. The rise time of the HP 54720 depends on which plug-in is used
and on whether real-time or equivalent-time sampling is used. Table 13-1
gives the rise time for various combinations of plug-in and sampling
technique.
Table 13-1 Rise Time Response
Plug-In
HP 54711A
HP 54712A
HP 54713A
HP 54721A
*** Not recommended for single-shot
13–38
Real-time Repetitive rise
rise time time
*** 233 ps
*** 318 ps
700 ps 700 ps
318 ps 318 ps