I have an undercounter AM, FM radio. When I plug the power supply to my
laptop
into an outlet in the same room, the FM signal gets very noisy. Upon further
investigation, I found that just plugging the ac cord into an outlet and
holding it near the laptop causes the noise. Also holding the laptop near
the power cord of the radio causes the noise. Holding the laptop near the
radio doesn't cause the noise.
So I think the laptop is generating noise that gets induced into the ac
line.
I tried wrapping the radio cord about (ten turns) through a (junkbox)
toroid without any help to the problem. The toroid is 3" in dia. It's
probably an iron laminate core
(low frequency) so may not be right for filtering the ac line.
My next step is to go to Radio Shack and try to purchase some clamshell
type
line filters.
Any ideas to eliminate this noise from the ac line?
Mike

"amdx" wrote in message
...
I have an undercounter AM, FM radio. When I plug the power supply to my
laptop
into an outlet in the same room, the FM signal gets very noisy. Upon
further investigation, I found that just plugging the ac cord into an
outlet and holding it near the laptop causes the noise. Also holding the
laptop near the power cord of the radio causes the noise. Holding the
laptop near the radio doesn't cause the noise.
So I think the laptop is generating noise that gets induced into the ac
line.
I tried wrapping the radio cord about (ten turns) through a (junkbox)
toroid without any help to the problem. The toroid is 3" in dia. It's
probably an iron laminate core
(low frequency) so may not be right for filtering the ac line.
My next step is to go to Radio Shack and try to purchase some clamshell
type
line filters.
Any ideas to eliminate this noise from the ac line?
Mike

Switch-mode power supplies are dirty ... lots of harmonic energy. The
interference is both radiated and sent back into the ac mains. A line
filter plus physical separation are the things to try.

"Charles"


Switch-mode power supplies are dirty ... lots of harmonic energy. The
interference is both radiated and sent back into the ac mains.


** But they don't usually generate energy in the FM broadcast band ( i.e.
88 - 108 MHz) - that is more likely to be due to the CPU and associated
high speed circuits inside the PC.




....... Phil

On Apr 29, 3:27*pm, "Charles" wrote:
"amdx" wrote in message

...





I have an undercounter AM, FM radio. When I plug the power supply to my
laptop
into an outlet in the same room, the FM signal gets very noisy. Upon
further investigation, I found that just plugging the ac cord into an
outlet and holding it near the laptop causes the noise. Also holding the
laptop near the power cord of the radio causes the noise. Holding the
laptop near the radio doesn't cause the noise.
So I think the laptop is generating noise that gets induced into the ac
line.
I tried wrapping the radio cord about (ten turns) through a (junkbox)
toroid without any help to the problem. The toroid is 3" in dia. It's
probably an iron laminate core
(low frequency) so may not be right for filtering the ac line.
*My next step is to go to Radio Shack and try to purchase some clamshell
type
line filters.
* Any ideas to eliminate this noise from the ac line?
* * * * * * * * * * * * * * * * * * * * * * * * * * * Mike

Switch-mode power supplies are dirty ... lots of harmonic energy. *The
interference is both radiated and sent back into the ac mains. *A line
filter plus physical separation are the things to try.

Some clocks for digital devices are intentionally "spread spectrum"
since the digital stuff doesn't care. This helps pass FCC type
acceptance. Spread spectrum is FM noise.

I remember that IC company IMI used to make SS clocks for digital
stuff. I wonder what ever happened to them.


http://findarticles.com/p/articles/m...28/ai_55000638
http://www.eetindia.co.in/ART_880048...T_7932edcb.HTM

On Apr 29, 5:55�pm, "amdx" wrote:
I have an undercounter AM, FM radio. When I plug the power supply to my
laptop
into an outlet in the same room, the FM signal gets very noisy. Upon further
investigation, I found that just plugging the ac cord into an outlet and
holding it near the laptop causes the noise. Also holding the laptop near
the power cord of the radio causes the noise. Holding the laptop near the
radio doesn't cause the noise.
�So I think the laptop is generating noise that gets induced into the ac
line.
�I tried wrapping the radio cord about (ten turns) through a (junkbox)
toroid without any help to the problem. The toroid is 3" in dia. It's
probably an iron laminate core
(low frequency) so may not be right for filtering the ac line.
� My next step is to go to Radio Shack and try to purchase some clamshell
type
line filters.
� �Any ideas to eliminate this noise from the ac line?
� � � � � � � � � � � � � � � � � � � � � � � � � � � �Mike

So, is it conducted noise or radiated noise?
The solutions are different.

If conduted, you can stop it with filters.
If radiated, you can stop it with shielding.
If both, you need both solutions.

It "sounds" to me like the radio is the problem, not the laptop, or
the laptop power supply.
Also, it's likely the weaker the FM station, the more noticable this
could be, depending of course on the type of detector the radio uses.
For AM, the noise could be horrendous!

An obvious thing to try: See if you have more than 1 branch circuit
available (in the kitchen, I presume??) If so, plug 'em in on
separate branches and see if this helps. (Note: Won't solve radiated
noise, but might help with conducted - and to me, that seems to be the
problem.)

Unless of course, I totally mis-read your post.

You can also do this test with an extention cord from another branch
circuit elsewhere in the dwelling. Even though the two branches have
at least some components in common, the separation can create enough
losses to lower the conducted emissions to acceptable levels.

Good luck.
-mpm

"Phil Allison" wrote in message
...

"Charles"


Switch-mode power supplies are dirty ... lots of harmonic energy. The
interference is both radiated and sent back into the ac mains.


** But they don't usually generate energy in the FM broadcast band ( i.e.
88 - 108 MHz) - that is more likely to be due to the CPU and associated
high speed circuits inside the PC.




...... Phil
Good call Phil,
Maybe I didn't say it clearly, but just plugging the ac cord in and
holding
it near the laptop induces noise on the power line. ( ac cord only, no
switching power supply attached)

"amdx"
"Phil Allison"
"Charles"

Switch-mode power supplies are dirty ... lots of harmonic energy. The
interference is both radiated and sent back into the ac mains.


** But they don't usually generate energy in the FM broadcast band (
i.e. 88 - 108 MHz) - that is more likely to be due to the CPU and
associated high speed circuits inside the PC.

Good call Phil,
Maybe I didn't say it clearly, but just plugging the ac cord in and
holding
it near the laptop induces noise on the power line. ( ac cord only, no
switching power supply attached)

** Validates my comment completely.



.... Phil

amdx wrote:
I have an undercounter AM, FM radio. When I plug the power supply to my
laptop
into an outlet in the same room, the FM signal gets very noisy. Upon further
investigation, I found that just plugging the ac cord into an outlet and
holding it near the laptop causes the noise. Also holding the laptop near
the power cord of the radio causes the noise. Holding the laptop near the
radio doesn't cause the noise.
So I think the laptop is generating noise that gets induced into the ac
line.
I tried wrapping the radio cord about (ten turns) through a (junkbox)
toroid without any help to the problem. The toroid is 3" in dia. It's
probably an iron laminate core
(low frequency) so may not be right for filtering the ac line.
My next step is to go to Radio Shack and try to purchase some clamshell
type
line filters.
Any ideas to eliminate this noise from the ac line?
Mike


Some of these undercounter AM/FM sets use the power line as an
"antenna". If you look closely you might find an antenna screw with a
metal clamp going around the power cord. If this is the case, you are
likely creating an extension of that antenna by plugging in one end of
the computer power cord and placing it near the laptop (which is
undeniably an noise source).

The fix is to get a separate FM antenna for the receiver and mount it
outside or at least some distance from your computer. Having a balanced
300 ohm type antenna might solve the problem.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

On a sunny day (Tue, 29 Apr 2008 16:55:12 -0500) it happened "amdx"
wrote in :

I have an undercounter AM, FM radio. When I plug the power supply to my
laptop
into an outlet in the same room, the FM signal gets very noisy. Upon further
investigation, I found that just plugging the ac cord into an outlet and
holding it near the laptop causes the noise.

Looks like that radio is using the main lead [power cord] as FM antenna.

So if you decouple that mains lead noise will _not_ get better.

Just keep the 2 separate.
Or, if the radio has an external antenna connector, use that.