HP 54645A User's And Service Manual page 151

Making Measurements with HP 54645A/D Oscilloscopes
To reduce the random noise on a signal
Use averaging to reduce noise on the displayed waveform.
3
Smoothing
The term "smoothing" is an oversampling technique used when the digitizer
sampling rate (200 MSa/s) is faster than the rate at which we're storing
samples into acquisition memory. For example, if the scope is sampling at
200 MSa/s, yet storing the samples at 1 MSa/s, it needs to store only 1 out of
every 200 samples. When smoothing is used, the slower the sweep speed, the
greater the number of samples that are averaged together for each display
point. This tends to reduce random noise on the input signal, which produces
a "smoother" trace on the screen.
Figure 5-7
256 Averages Used to Reduce Noise
5-10
, then press Average.
Press
Display
Notice that appears in the status line.
Av
Toggle the # Average softkey to select the number of averages that best
eliminates the noise from the displayed waveform.
Av in the status line indicates how much of the averaging process is
finished, by turning to inverse video as the oscilloscope performs
averaging. When the number of averages is higher, more noise is
removed from the display. However, the higher the number of
averages, the slower the displayed waveform responds to waveform
changes. You must choose between how quickly the waveform
responds to changes and how much you want to reduce the displayed
noise on the signal.