USB device interfering AM radio

[Man-wai Chang]

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

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

--
@~@ Might, Courage, Vision, SINCERITY.
/ v \ Simplicity is Beauty! May the Force and Farce be with you!
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[Jeff Liebermann]

On Sun, 20 Mar 2011 21:15:04 +0800, Man-wai Chang
<toylet.toylet@gmail.com> 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 jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Meat Plow]

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.

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

The adapter may be a switch mode supply and you are hearing a harmonic
spur of the frequency the supply uses to function on. A switch to non-
switch mode supply would cure your problem.



--
Live Fast Die Young, Leave A Pretty Corpse

 

[Man-wai Chang]

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.

A switch to non-switch mode supply would cure your problem.

Thank you both

--
@~@ Might, Courage, Vision, SINCERITY.
/ v \ Simplicity is Beauty! May the Force and Farce be with you!
/( _ )\ (x86_64 Ubuntu 9.10) Linux 2.6.38
^ ^ 15:07:01 up 2 days 18:41 0 users load average: 1.12 1.12 1.13
不借貸! 不詐騙! 不援交! 不打交! 不打劫! 不自殺! 請考慮綜援 (CSSA):
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa