Nope, they must by necessity of the use of switching PS's have some AC elements involved too. No switching is pure and perfect, and at every switching cycle there will be spikes, surges, and reversals of current flow because of that, creating AC even if only minimally. This is why I said RFI is inherent in switching PS's because it is. RF cannot be generated with a DC current- that takes an AC current. RFI exists here, therefore at least some AC must be present for that to occur. RF doesn't behave like DC, therefore premises which apply to DC do not apply equally here. When USB was developed, it was known, intended to, and used switching PS's, although other sources could be used too. By that time, switching PS's were the only ones being used save for specialty applications because they were far more efficient, less costly, smaller, lighter, and worked excellently in their intended applications. The developers knew there would be some AC present, but it wasn't significant enough to be a problem with USB's intended use. Now we're using USB in uses far beyond it's design intents, and now we're having problems. You cannot totally choke or filter out all AC in a DC current. No technology exists to do this at that level. And that is even moreso when that AC is at RF frequencies. So the RF and AC is there in USB too, just as they knew it would be.
In RF engineering you can use a coil to amplify or to choke a given frequency. There are many elements involved with that including cosmic radiation if you care to measure it that carefully (which has been done). Any loop or coil you make has the potential to amplify that RF, therefore the best and simpliest way to avoid that type of amplification is to avoid coiling or looping. Yes, you might also create a choke, but that is the less likely outcome of the two, as RF tends to reflect back to the originating coil from the ones adjacent to it, thus gaining energy with each cycle which increases the tendency until a saturation point is reached where the coil can carry no more current. Creating a choke takes a more exact sizing and spacing of the coils, and even then it acts as a choke only at one frequency and to a lesser degree with the harmonics of that frequency. And in our usage, there's a car body and wiring and computer signals all adding to the effect. In fact, if you were to have a pure DC current to start with it would not be pure at the load because of those interactions unless you shielded it 100% from that, and shielding cannot be done at that level either.
Even if we power our cam with a battery which isthe only known pure DC power source, you will still find AC on the USB cable. This is because the cam is not a purely resistive load, and it too will influence the current flowing on the USB cable, reflecting some of the current back cyclically as AC or steadily in a standing waveform which will not remain static as the load changes and creating AC in the process. This is all simple stuff to a radio geek like me, but it's far beyond what electrical engineering covers in it's normal sense and usage, and this is an RF game, not a DC one.
Cordially,
Phil
