Fixing FM-band RFI from homebuilt desktop PC
April 4, 2021 8:11 AM   Subscribe

A few months ago I built a new desktop PC that is causing FM interference with my stereo receiver setup. This interference (buzzy static) only occurs when the PC is on. I did not have this problem with my previous PC build (from 2011). I need to solve this. A few more details inside.

The stations I want to receive are fairly weak at my location. Listening in mono, the fidelity is normally excellent. A strong nearby station is not affected by this interference. Nor are any other functions of the stereo (CD, tape, etc).

The interference seems to be lessened, but not eliminated, by touching the computer case or the back of the motherboard where the connectors are. It makes me think of a grounding problem but the outlet ground tests good.

The case I used is more open and airy than anything I've had before, and it also has one glass side. Should this be a problem? How can I test it? I can change to a different case if necessary.

The FM antenna resides in a window about 8 feet from the computer. It's a directional antenna, set up for 3 stations in that direction.

I know I could be streaming the same FM content, but I don't want to do that. Moving the antenna and PC farther apart or getting an outdoor antenna might also work, but would be inconvenient or expensive. I really just want to find a way to quiet the interference from the computer. My old computer in the same location was never a problem.

I can give a list of parts I used for the PC if that would be helpful, but it's all mainstream stuff.

Any ideas on how I can go about fixing this? Thanks in advance.
posted by DarkForest to Computers & Internet (7 answers total)
Best answer: Barring some very weird stuff happening, I think a fan is the only thing in a computer that could cause this. Disconnect each of your case fans one at a time and test. It probably isn't the CPU or GPU fans, but that is a possibility too.

Once you figure out which fan it is, you can determine whether there's something up with the wiring or if you could just replace that fan. Most of them are pretty cheap, no?

Anyway, that's where I'd start.
posted by Krawczak at 8:48 AM on April 4, 2021

Best answer: Try covering the window in the case with aluminum foil. Make sure the foil is grounded to the case. If this reduces the interference, the RF signal is radiating through the case window.
posted by gnossos at 10:45 AM on April 4, 2021

Best answer: I think a fan is the only thing in a computer that could cause this.

You forgot a 'not' in that sentence.
From an RFI viewpoint small DC fans are about the least problematic components in a PC, even if they are speed-regulated and brushless. Whatever RFI they emit is at most in the tens of kHz range for its base frequency, and even the 7th harmonic will not even be in the low MHz range plus it'll be extremely weak. All the OTHER parts in a PC emit frequencies that are much higher; for instance a 33MHz PCI bus has its third harmonic at 100MHz, which is right in the middle of the FM band.

DarkForest: the ground connection at the wall socket is OK as you say, and through that the power supply's enclosure should also be grounded. But is there good contact between the PSU and the computer case?
posted by Stoneshop at 12:31 PM on April 4, 2021 [2 favorites]

Oh of course, I completely spaced that we're talking about DC motors, never mind!
posted by Krawczak at 12:51 PM on April 4, 2021 [1 favorite]

Best answer: I feel there are two ways to resolve this:

1) Add a TV antenna to the roof, which will also help with your FM reception, IIRC if you hook up your radio to it

2) Try rotating the case to see if a particular orientation helps. Somehow, I feel the ever popular "side window" may be to blame for this one, but I don't have any solid reasons to suspect so other than that seem to be the biggest hole any EFI can "leak" from.
posted by kschang at 1:17 PM on April 4, 2021

Response by poster: It seems like it must have been a poor connection somewhere. I was going to put the case on its side and pull the fan connectors as a start. But as I was turning it on its side, the interference stopped. I rebooted the computer on its side a couple times as the interference was usually particularly bad during boot up, but on its side there was no problem. I set up back upright, and still no problem. So something seems to have fallen into place. If the problem crops up again later, I'll take the sides off and make sure everything is tight.

I can't believe I've gone months like this and I'm very happy to have it solved. Thanks for all your answers.
posted by DarkForest at 1:49 PM on April 4, 2021

I was going to suggest that perhaps it's not wireless interference, but rather, POWER NOISE, and recommend you move your radio to a different power outlet, but if it's solved, it's solved. :)
posted by kschang at 7:26 PM on April 4, 2021

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