I was hoping someone could help me better understand what I am measuring in IQ Analyzer Mode.
My signal going into the SA is a pulsed 50-MHz RF signal. The pulses are 50ms wide with a 100ms period. See attached oscilloscope image of the signal (signal.jpg). On the oscilloscope I verify that every pulse starts with the same phase.
I use the pulse as an EXT1 trigger for a SA measurement, and I use trigger holdoff 70ms so I only take one measurement per pulse, and each measurement should start at the same time. I use a trigger delay of 1ms so the entire measurement is taken with the pulse on.
In the attached video phase.mp4 (sorry for poor quality), the top graph is the phase "FETC:SPEC 10" and the bottom graph is the time domain signal "FETC:SPEC 2" (see IQ Analyzer Mode Reference page 416).
As you can see, the phase measurement randomly switches between 3 versions. I expect to see the phase stay constant like attached "goodphase.jpg", where it is jumping between -0.8 and 2.4 (difference is pi). But the other version "badphase.jpg" looks pretty much like random noise to me.
Does this reading make sense to you? Why isn't every RF pulse giving me the same measurement? How should I interpret this?
Thanks for your help,