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Triggering
TDS 620A, 640A & 644A User Manual
2Ć15
H Automatic Ċ this trigger mode (auto mode) lets the oscilloscope acĆ
quire 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 oscilloscope 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, auto mode is
useful in observing signals where you are only concerned with monitoring
amplitude level. 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.
When a trigger event is recognized, the oscilloscope 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 time to help ensure a stable display.
For example, the trigger signal can be a complex waveform with many
possible trigger points on it. 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 2Ć2). 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 digitizing oscilloĆ
scope to trigger on the correct edge, resulting in a stable display.
Holdoff