rfi from a silverstone st30nf

walkingjohn · Post by **walkingjohn** » Sat Jul 14, 2007 2:23 pm

Hi all,

My new nightjar fanless psu is causing a whole lot of interference with TV and radio reception (I know it's the silverstone, because when I switch to an older generic power supply, there's zero speaker crackle, even though this ps is outside the case). I'm curious to hear if others have had this problem.

More importantly, though, how can I shield this thing? Can anyone help? It's lovely and quiet--well, proximally quiet--and expensive, and I'd like to make it usable.

Confession: the case is missing most of its metal (long story--airflow and such: I've been lurking and modding), but I've tried to make a faraday cage with aluminum window-screening. The screen is fixed to grounded metal at many points, but there are a number of thin gaps. If I understand the physics right, that shouldn't matter--am I wrong?

Mr Evil · Post by **Mr Evil** » Sat Jul 14, 2007 4:07 pm

To be effective, any holes in the shield need to be much smaller than the wavelength to be blocked. ATX power supplies might have a switching frequency of 100kHz, which gives a wavelength of about 3km. There will also be higher frequency harmonics present, but the wavelength is still going to be pretty long, so even quite large holes shouldn't be too much of a problem.

A bigger problem is that bare aluminium quickly forms a coating of oxide on the surface, which is a good electrical insulator, making it very hard to connect the parts of the shield together. Bolts are good, along with those jagged metal washers (I don't know what they're called).

You might try plugging the computer into a filtered mains power strip, to reduce any noise that is conducted rather than radiated.

walkingjohn · Post by **walkingjohn** » Mon Jul 16, 2007 8:38 am

Mr Evil wrote: You might try plugging the computer into a filtered mains power strip, to reduce any noise that is conducted rather than radiated.

Thanks--I hadn't thought of that. Will give it a try.

Update: I know at least one other person has had similar problems, so here's what (little) I've found...

1. Using a power bar with rfi filtering didn't help.

2. Fiddling with the power cable helps, though not at every frequency at once--so perhaps that's where the radiation's coming from. Makes a certain amount of sense, since the cable's not shielded to my knowledge.

3. Wrapping the power cable in aluminum foil eliminated static on some but not all frequencies (and made the cable look like a prop from a bad sci-fi movie).