Hello Feng,
You did not state whether you are using the API or using our supplied software. If you are using our software then you can setup video triggers in zero-span mode using the control panel. If you are using the API and retrieving IQ data from the device directly then video triggering is just a function of checking each IQ sample for you trigger level.
If this does not answer your question, please try to restate your questions.
Regards,
A.J.
