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Triggering
TDS 620A, 640A, & 644A User Manual
Automatic — this trigger mode (auto mode) lets the oscilloscope 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 oscilloscope forces a trigger anyway. The
length of time it waits for a trigger event depends on the time base set-
ting.
Be aware that auto mode, when forcing triggers in the absence of valid trig-
gering events, does not sync the waveform on the display. In other words,
successive acquisitions will not be triggered at the same point on the wave-
form; 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 possi-
ble 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 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 digitizing oscilloscope to
trigger on the correct edge, resulting in a stable display.
Holdoff