238
Logic triggers are special-purpose triggers that are primarily used with digital logic signals.
Two of the types, Pattern and State, trigger the instrument based on the Boolean operator you
select for the trigger sources. A third type, Setup and Hold, triggers when data in one trigger
source changes state within the setup and hold times that you specify relative to a clock in
another trigger source. Logic triggers are available on the main trigger only.
Communication triggers (available only with Option SM) are designed to work with
communication masks and standards. Mask testing automatically uses Communication
triggers.
Video triggers are used to trigger the instrument on specified fields or lines of a video signal.
You can use one of several preset video signal formats or set a custom format.
Trigger Modes
The trigger mode determines how the instrument behaves in the absence of a trigger event:
Normal trigger mode enables the instrument to acquire a waveform only when it is triggered.
If no trigger occurs, the instrument does not acquire a waveform, and the last waveform
record acquired remains "frozen" on the display. If no last waveform exists, no waveform is
displayed.
Auto trigger mode enables the instrument 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 time out, the instrument forces a trigger. The length of time it waits
for a trigger event depends on the time base setting.
Auto mode, when forcing triggers in the absence of valid triggering events, does not synchronize
the waveform on the display. In other words, successive acquisitions are not triggered at the same
point on the waveform; therefore, the waveform will appear to roll across the screen. If valid
triggers occur, the display will become stable.
Trigger Holdoff
Trigger holdoff can help stabilize triggering. When the instrument 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. Adjust holdoff to obtain stable
triggering when the instrument is triggering on undesired trigger events.
A digital pulse train is a good example of a complex waveform. 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 instrument to trigger on the correct edge, resulting in a stable display.
Summary of Contents for TDS5000B Series
Page 36: ...36 AFTDS Differential Signal Adapter...
Page 280: ...280 Blackman Harris window...
