Help, Laptop causes FM interference

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

Reply to
amdx
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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.

Reply to
Charles

"Charles"

** 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

Reply to
Phil Allison

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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.

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Reply to
Simon S Aysdie

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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

Reply to
mpm

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)

Reply to
amdx

"amdx"

** Validates my comment completely.

... Phil

Reply to
Phil Allison

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"©
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Reply to
RFI-EMI-GUY

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

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.

Reply to
Jan Panteltje

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