Agilent Technologies 54622A User Manual page 65

Triggering the Oscilloscope
To select a trigger mode: Auto Level, Auto, Normal
Normal mode
Use Normal trigger mode for low repetitive-rate signals. In this mode, the
oscilloscope has the same behavior whether the acquisition was initiated by
pressing Run/Stop or Single.
When the trigger event is found, the oscilloscope will fill the post-trigger buffer
and display the acquisition memory. If the acquisition was initiated by Run/Stop,
the process repeats. The waveform data will be scrolled onto the display as it
is being acquired.
In Normal mode the oscilloscope must fill the pre-trigger buffer with data before
it will begin searching for a trigger event. The trigger mode indicator on the
status line flashes to indicate the oscilloscope is filling the pre-trigger buffer.
While searching for the trigger, the oscilloscope overflows the pre-trigger buffer;
the first data put into the buffer is the first pushed out (FIFO).
In either Auto or Normal mode, the trigger may be missed completely under
certain conditions. This is because the oscilloscope will not recognize a trigger
event until the pre-trigger buffer is full. Suppose you set the Time/Div knob to
a slow sweep speed, such as 500 ms/div. If the trigger condition occurs before
the oscilloscope has filled the pre-trigger buffer, the trigger will not be found.
If you use Normal mode and wait for the trigger condition indicator to flash
before causing the action in the circuit, the oscilloscope will always find the
trigger condition correctly.
Some measurements you want to make will require you to take some action in
the circuit under test to cause the trigger event. Usually, these are single-shot
acquisitions, where you will use the Single key.
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