Agilent Technologies 4000 X-Series User Manual page 198

10
Triggers
The trigger circuits look for the position of the start of Vertical Sync to
determine the field. But this definition of field does not take into
consideration the phase of the reference subcarrier. When Field 1 is
selected, the trigger system will find any field where the vertical sync
starts on Line 4. In the case of NTSC video, the oscilloscope will trigger
on color field 1 alternating with color field 3 (see the following figure).
This setup can be used to measure the envelope of the reference burst.
Figure 32
If a more detailed analysis is required, then only one color field should be
selected to be the trigger. You can do this by using the Field Holdoff softkey
in the Video Trigger Menu. Press the Field Holdoff softkey and use the Entry
knob to adjust the holdoff in half- field increments until the oscilloscope
triggers on only one phase of the color burst.
A quick way to synchronize to the other phase is to briefly disconnect the
signal and then reconnect it. Repeat until the correct phase is displayed.
198
Triggering on Color Field 1 Alternating with Color Field 3
Agilent InfiniiVision 4000 X-Series Oscilloscopes User's Guide