Hello Master Pine.
Ah yes, GND. So, that was my first investigations. At this point, I have pulled the Linino out of the original circuit to test it with just the PS and the Linino and have fastidiously connected all the GNDs. I then connected the ground bus to the GND on the scope and also tested ground continuity all the way back to the mains and they are all common, i.e. I can touch a ground on any part of the circuit, scope, PS, all the Linino GND pins and even the neutral to GND at the mains and they are connected with no more than 2 Ohms of resistance between points.
With the Linino alone, I get several burst per second. When I attach the DogUSB, it gets much noiser with what looks like data leaking into the supply rails.
Finally, I did power this from a bench supply -- no ground issues there either.
I am perplexed because looking at the Linino schematic we should not be getting this noise. What do you think about the voltage drop across the zener at the Linino? (I've included part of the Linino schematic.) Do you think that the diode protection is resulting in a voltage drop across the diode that serves a platform for the noise to not drain to ground?
I've also included an example of the Linino only noise. Scale is 100mV.
Again, I appreciate your help on this.