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Shielding components from EMI/RFI... Help please

A recent experiment with a product designed to reduce EMI/RFI left me curious about other ways to reduce EMI/RFI in my system. In the past ten days, I've stepped onto a slippery slope, at the bottom of which is surely some kind of insanity...

I've been experimenting with copper plates in an effort to absorb, deflect, diffract, and block EMI/RFI. I've tried copper plates under components, on top of components, and inside components.

This is the point where you tell me I don't know what I'm doing and I'm likely to short circuit something and/or electrocute myself. Consider me duly warned. This is also the point where you tell me to get some balanced interconnects, or at least to get some shielded interconnects for Chrissake. Consider me duly informed. Moving on...

I'm hoping you can help me make the most of this experiment, and help me avoid killing a component or myself. My strategy so far has been to:

1. Place copper plates at locations that generate a lot of EMI/RFI, e.g., components with switching mode power supplies or high frequency clocks. The system has a total of 3 SMPS and 3 clocks.

2. Place copper plates at locations that are vulnerable to EMI/RFI, e.g., under the amp, near the transformer.

3. Place copper plates inside noisy components -- in particular, my Meridian G68 preamp/processor. I've begun to build 2 partial Faraday cages, one for the SMPS, and one for the analog output stage.

4. Ground the copper plates either to the component chassis (when plates are used inside a component) or to an independent ground point (when plates are used above/below a component).

Has anyone tried this sort of thing?

Bryon
bryoncunningham
kijanki's avatar
kijanki
4,428 posts
 
"I've been reading that copper is effective at shielding high frequency RFI, but not particularly effective at shielding low frequency EMI"

It is because copper, being non-magnetic, does not shield against electromagnetic wave. In cable this electromagnetic wave induces current but because of skin effect it travels on the outside of the cable - shield . Same happens with copper plates around components. Skin effect won't work at lower frequencies where copper becomes useless against magnetic field of 60Hz transformer or even 50kHz from switching power supply.

In case of low frequency radio interference you need at least 1/10 of the wavelength antenna to "receive" interference - not likely. You are more susceptible to direct magnetic fields like leakage field from power transformers.

MuMetal is good suggestion - it works much better than plain steel (permeability of steel is few thousand while MuMetal reaches 100,000). It comes in different permeability because higher permeability Mu Metal is easier saturated by magnetic field. In such cases sandwich of lower and higher permeability MuMetals gives best results. Your magnetic fields are most likely very weak but just in case don't place MuMetal close to the source (transformer etc.). Excessive bending of MuMetal is not advised (has to be re-annealed).
jea48
4,459 posts
 
Bryon,

You might want to do some research on the archives of Tweakers' Asylum regarding the use of ferrous, magnetic, materials. There may be better ways to shield for EMI.
Search shielding for EMI

http://www.hottconsultants.com/pdf_files/ground.pdf

http://www.q-audio.com/grounding.pdf
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bryoncunningham OP
890 posts
 
Jea - Thanks for the advice. I will poke around on Tweakers and see what I discover.

Kijanki - Thanks for the help. You are always a good source of information. Much appreciated.

MuMetal is good suggestion - it works much better than plain steel.

I already ordered custom cut regular steel from an online retailer. :-( You think using regular steel is a waste of time?

Your magnetic fields are most likely very weak but just in case don't place MuMetal close to the source (transformer etc.).

Again, what if I use the regular steel I already purchased? It's T-304, which I believe is non-magnetic. If that's true, do I still have to be careful how close I get it to the power supply? How close is too close?

Bryon
bryoncunningham OP
890 posts
 
Now that I think about it, if the steel I purchased is indeed non-magnetic, will it work for shielding EMI at all?!

bc
kijanki's avatar
kijanki
4,428 posts
 
T-304 won't shield against low frequency magnetic field. It might protect against it if you build Faraday Cage but even then it is very limited. I'm no expert on Faraday Cage but as far as I remember it is used mostly as electrostatic shield. It works against magnetic field but poorly at lower frequencies.

We don't even know what is the nature of the noise pickup. If switching power supply has fast transients (high frequency) even non-magnetic shield would help cutting on capacitive coupling and creating losses (eddy currents) with magnetic field. I would stay on the course, since you already ordered shields. It will give you shielding at high frequencies allowing to isolate the problem.

I absolutely agree with Al's assessment on the Cat6 improvement. Jitter is not only function of the signal noise (not likely since it is buffered) but also system noise (receiver threshold noise) that is influenced by noise injected by current induced in the cable that finds return to ground thru the system. Ethernet uses differential signaling that cuts on common mode noise but it still couples high frequencies thru input capacitance to signal ground. Currents traveling on the signal ground are system noise. Judging by improvements you experienced your noise pickup might be of high frequency and your non-magnetic shields will work.
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