On Sun, 20 Mar 2011 21:15:04 +0800, Man-wai Chang
wrote:

I just discovered this. And I also found that the AC to USB power
adaptor does not have a metal ground pin.

It's double insulated and therefore does not need a protective
grounded plug.

So would this AM interference be gone after getting a better AC to USB
power adaptor?

Yes. What you're hearing are harmonics of the 20-50Khz switching
frequency in the power supply. These are suppose to meed FCC Part 15
incidental and conducted radiation specifications. Except for name
brand manufacturers, most of these wall warts do not meet the specs
(by my measurements). There's no way to easily predict which wall
wart is any better than any other, leaving you with trial and error to
make your selection. Fortunately, USB power adapters are cheap and
common, so you should be able to find one that's fairly clean (and
return the others).

Also, note that your unspecified model AM radio may also be
contributory. My guess(tm) is that you have both the USB power
adapter and the radio plugged into the same outlet or power strip.
Conducted radiation, through the power lines is another component of
the problem. Your radio may be susceptible to junk pickup from the
power lines, or for really cheap junk, use the power cord as an
antenna. Clamp on ferrite beads (more like ferrite blocks) are
available to block conducted radiation and pickup.

http://en.wikipedia.org/wiki/Electromagnetic_compatibility
http://www.radiolabs.com/Articles/amnoise.html
http://www.stevelarkins.freeuk.com/images/computer_rfi_solution2.jpg

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558