[gary]

Hi,

Keep in mind that USB itself has two lines devoted to carrying power and
ground through the cable.

Also keep in mind that USB uses differential signaling, so *in theory* common
mode noise gets cancelled out. Furthermore, high speed USB cables have
a braided shield to help with both radiation and assist with immunity.

*In practice*, for reasons I won't go into here, common mode noise problems
can still occur on USB cables.

What is really important is that the designer of the PC and the USB peripheral
have incorporated best design practice in their designs. For example,
the USB lines should have had appropriate protection filtering at the PCB
level at either end and the metal shields terminated correctly to the appropriate
'dirty' ground points.

With the high signaling speeds of modern digital systems including USB
inter-connects, PCB traces and cables become antennae which can both transmit
and receive RF radiation. This poses challenges for the designers of such systems.

Though noise from AC systems can be an issue, don't think for one moment
that powering the system from a battery will necessarily eliminate any problems.

Again, the culprit can actually be the high speed data traces on the PCB itself
being transmitted back out along the power lines that came into the unit.
Again, this is where best design practice can make all the difference and
on a properly designed peripheral, conduction of noise back through the
power lines should have been designed to be within regulatory limits - but
it may not have been. In other words, one bad apple can spoil the basket.
Only buy peripherals that carry a C-Tick marking (or CE). That doesn't
mean they won't cause a problem, but you are at least assured they were
probably tested.

Best Regards

Gary