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Re: EMI

Posted by John Swenson (A) on February 9, 2007 at 13:54:35

In Reply to: EMI posted by Christine Tham on February 7, 2007 at 18:16:51:

I have done some tests, I hooked up a 2 inch piece of wire to the probe on my 0-40MHz spectrum analyzer and moved it around components, wires etc. I get radically different amounts and spectrum of EMI from different devices and in different directions from the same device.

Laptops as a group are pretty good EMI wise (at least the ones I tested), but every "quiet PC" I've tried has leaked like a sieve. Thats four separate PCs in either fanles or very quiet fan systems that are designed to be put on racks with other components. Two were commercial systems used as is from the factory and two were home built systems using cases designed for the purpose. I wouldn't put any of those in my rack.

I also did some testing of a couple other devices such as DVD playes, the SB3, power amp etc. These varied quite a bit. The 4 DVD players I tried were not too bad, about on par with the laptops. The expensive CD transport I have was terrible, maybe thats part of why I never liked it very much. Regular analog stereo components also emit EMI but most of that is power supply related, 60, 120, 180 Hz components. Even though the power transformers are burried inside a metal box I could still eaily pick up the fields from them.

On another question of yours the cables do radiate EMI, but it was usually far less than the boxes themselves put out. I can get the same level of sound degradation from those silent PCs without any connections to them other than the power cord.

I also tried el cheapo shielding (ie wrapping in aluminum foil) and it does actually help quite a lot, but its not really very good to the devices due to air flow restrictions. (its also really hard to see a display when its wrapped in aluminum foil, I need to get that formula from Scotty for transparent aluminum!)

I think its rather intrigueing that the mass market "consumer" items seem to have much lower EMI. They probably DO meet FCC class B, but the other items don't.

The tests I did were completely uncalibrated so I have no clue as to where they stood with regards to FCC regs, they were just to get some idea of the relative amounts and spectra of stuff coming out of some of the boxes in my system.

John S.

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