I'm trying to capture IQ samples from a PXA using Labview (I can download them with Labview). My source is a PSG generating a digital modulation (4QAM or BPSK).
The point is that, if I set the PXA using Basic->IQ Waveform->Display->I/Q Waveform I'm able to distinguish the samples. I'm using an scope to monitor de Q samples form the BNC output in the PSG. So I can actually see the bits.
Nevertheless, the blue and yellow lines in the PXA representing I and Q are not still. They should the same be from trigger to trigger as happens in the scope given that the PN23 sequence repeats all the time.
When I download the samples to labview and plot the IQ samples I see that the constellation spins all the time. It's never still.
The issue you are seeing is probably due to the fact IF Gain is set to “Autorange”. Auto range was the default with earlier PXA SW, but has since been changed in later SW to default to Low Gain, which solves the problem.
Early PXA SW Rev A.8.54 or earlier SW had this problem. SW A.10.04 or newer fixes the problem. However, it is possible for the user to manually select Autorange by going to I/Q analyzer mode, press: Meas Setup, More, Advanced, More, IF Gain and manually select Autorange. You can use this key sequence to select the Low Gain setting to assure the problem you are seeing goes away.