Triggering on Waveforms
TDS 684A, TDS 744A, & TDS 784A User Manual 3–51
in another trigger source. Logic triggers are available on the main trigger system
only.
Video (available as option 05) is a special trigger used on video circuits. It helps
you investigate events that occur when a video signal generates a horizontal or
vertical sync pulse. Supported classes of video triggers include NTSC, PAL ,
SECAM, and high definition TV signals.
The trigger mode determines how the oscilloscope behaves in the absence of a
trigger event. The oscilloscope provides two trigger modes, normal and automatic.
Normal trigger mode enables the oscilloscope to acquire a waveform only when
it is triggered. If no trigger occurs, the oscilloscope will not acquire a waveform.
(You can push FORCE TRIGGER to force the oscilloscope to make a single
acquisition.)
Automatic trigger mode (auto mode) enables the oscilloscope 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 oscilloscope forces a trigger anyway. The length of time it
waits for a trigger event depends on the time base setting. (Auto mode triggering
is not available in InstaVu mode (TDS 700A models only); see Incompatible
Modes on page 3–45.)
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 the oscilloscope recognizes a trigger event, it 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.
A digital pulse train is a good example of a complex waveform. (See Fig-
ure 3–31.) Each pulse looks like any other, so many possible trigger points exist.
Trigger ModesTrigger Holdoff