Tektronix, Inc. P6240, Руководство пользователя

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Triggering
TLS 216 User Manual
2–15
Trigger Modes
The trigger mode determines how the logic scope behaves in the absence of a
trigger event. The logic scope provides two different trigger modes, normal and
automatic.
Normal trigger mode allows the logic scope to acquire a waveform only when it
is triggered. If no trigger occurs, the logic scope will not acquire a waveform.
(You can push FORCE TRIGGER to force the logic scope to make a single
acquisition.)
Automatic trigger mode (auto mode) allows the logic scope to acquire a
waveform even if a trigger does not occur. Auto mode uses a timer that starts
after a trigger event occurs. If another trigger event is not detected before the
timer times out, the logic scope forces a trigger anyway. The length of time it
waits for a trigger event depends on the time base setting.
Be aware that auto mode, when forcing triggers in the absence of valid triggering
events, does not sync the waveform on the display. In other words, successive
acquisitions will not be triggered at the same point on the waveform; therefore,
the waveform will appear to roll across the screen. Of course, if valid triggers
occur the display will become stable on screen.
Since auto mode will force a trigger in the absence of one, you can use it to
monitor signals the amplitude level of which is your only concern. Although the
unsynced waveform may “roll” across the display, it will not “freeze” as it would
in normal trigger mode. Monitoring of a power supply output is an example of
such an application.
Holdoff
When a trigger event is recognized, the logic scope disables the trigger system
until acquisition is complete. In addition, the trigger system remains disabled
during the holdoff period that follows each acquisition. You can set holdoff to
help ensure a stable display.
For example, the trigger signal might be a complex waveform with many
possible trigger points. Though the waveform is repetitive, a simple trigger
might get you a series of patterns on the screen instead of the same pattern each
time.
Digital pulse trains are good examples (see Figure 2-2). Each pulse looks like
any other, so many possible trigger points exist. Not all of these will result in the
same display. The holdoff period allows the logic scope to trigger on the correct
edge, resulting in a stable display.