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A more technical question than usual from me.

Is anyone familiar with the distortions that either power supply variation, or intentional amplifier pre-distortion (DPD) can create on I/Q diagrams when testing WLAN transmitters.

I have seen both circular and square data-point arrangements within the tolerance-ring limits for the individual symbols.   The doughnut shaped patterns are supposedly caused by power supply variations, but what would cause the "clouds" of data points to become squares.

In addition, there may be no other obvious compression, or phase shift evidence in the overall diagrams - and the average EVM numbers are within limits.

Data points for the 2.4 GHz seem to be much more affected than those of 5 GHz channels.  From an EVM number standpoint, this appears to be a positive development - but I'd like to understand the cause(s). 

Thanks.