HP 54645A, User'S And Service Manual

Page: 62 / 301

Selecting Trigger Modes and Conditions
The trigger mode affects the way in which the oscilloscope searches
for the trigger. Figure 3-1 shows the conceptual representation of
acquisition memory. Think of the trigger event as dividing acquisition
memory into a pre-trigger and post-trigger buffer. The position of the
trigger event in acquisition memory is defined by the time reference
point and the delay setting.
Acquisition Memory
To select a trigger mode: Auto Lvl, Auto, Normal
1
Press
Mode/Coupling
.
2
Select the Auto Lvl
,
Auto
, or
Normal
trigger softkey.
Auto Lvl and Auto modes
Use the Auto modes for signals other than low-repetitive-rate signals. To
display a dc signal, you must use the auto trigger mode since there are no
edges on which to trigger.
Auto Lvl mode is the same as Auto mode with an automatic trigger level
adjustment. The oscilloscope looks at the signal level on the signals, and if
the trigger level is out-of-range with respect to the signal, the scope adjusts
the trigger level back to the middle of the signal.
When you select Run, the oscilloscope operates by first filling the pre-trigger
buffer. It continues to flow data through this buffer while it searches for the
trigger. When a trigger is found, the pre-trigger buffer will contain the events
that occurred just before the trigger. If no trigger is found, the oscilloscope
generates a trigger and displays the data as though a trigger had occurred.
Figure 3-1
3-3