HP 54620A User's And Service Manual page 132

Figure 36
Acquisition Memory
Normal Mode In Normal trigger mode, the analyzer behaves the same
when the acquisition was initiated by pressing either Run or Single. The
analyzer begins filling the pre-trigger buffer with data. As soon as that
buffer is full, the analyzer will begin searching for the trigger event, and
will flash the trigger condition indicator. While searching for the trigger,
the analyzer overflows the pre-trigger buffer, with the first data put into
the buffer being the first pushed out (FIFO).
When the trigger event is found, the analyzer will fill the post-trigger buffer
and display the acquisition memory. If the acquisition was initiated by
Run/Stop, the process repeats.
Auto Mode The analyzer fills the pre-trigger buffer, then searches for
the trigger event for a predetermined interval. This interval is a function
of the Time/Div setting, but is never less than 40 ms. If no trigger is
found, the analyzer forces a trigger and displays the data as though a
trigger occurred. The subsequent behavior depends on whether the
acquisition was initiated by pressing Single or Run:
•
Single—the analyzer will fill acquisition memory, stop, and display the
results.
•
Run—the analyzer will fill the pre-trigger buffer after drawing a trace.
When the pre-trigger buffer is full, the analyzer repeats the search for a
trigger. If no trigger is found, a free-running trace is displayed, much like
that of an oscilloscope on auto trigger.
In either Auto or Normal mode, the trigger may be missed completely under
certain conditions. This is because the analyzer 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 always
occurs before the analyzer has filled the pre-trigger buffer, the trigger will not
be found.
Ensuring Accurate Measurements
Time base and Acquisition
131