Shielding components from EMI/RFI... Help please

Hi Bryon,

Good find. Whatever Dr. Johnson says, is so! He is one of the world's leading experts on high speed digital signal transmission.

As it happens, I took his course on high speed digital design about 15 years ago, in connection with my job. This is the associated textbook. You'll be amused at its sub-title, "a handbook of black magic." As you certainly realize at this point, grounding and shielding are among THE most arcane and mysterious aspects of electrical engineering, with problems often being resolved by not much more than blind trial and error. I, btw, am by no means an expert in that area.

With respect to the second paragraph of your post just above, where you said "correct me if I am wrong," I believe that everything you said is correct.

With respect to the apparent paradox cited at the end of your post, I believe that two things need to be considered.

First, for shielding to be effective at high frequencies, as Dr. Johnson indicated the shield should be grounded at both ends. But I believe that the key element of what he is referring to by "grounding" is a connection at each end between the shield and the metallic structure of the component, rather than a connection to some external ground point. I believe that circulation of noise currents from the cable shield into the metallic structure of the components will dissipate their energy significantly, although perhaps less so in the case of the network switch due to its small size.

Second, concerning the improvement you noted when connecting the 14 gauge ground wire to the network switch, my speculation is essentially as I commented yesterday:

Lower frequency grunge presumably was also present, perhaps associated with the computer's switching power supply, power line distortion, emi pickup, etc, the effects of which may not have been entirely eliminated by the reclocker. Your ground connection is presumably a much better conductor at those lower frequencies than at the very high signal-related frequencies ...

You were probably reducing the amount of low frequency noise that had been present at that point (the sources of that noise being unrelated to the ethernet signals themselves), that was making its way through the circuit grounds downstream, resulting in jitter that was not being entirely eliminated by the reclocker.

All that is obviously fairly speculative, but those are the only explanations I can think of that seem to fit all the facts.

Best,
-- Al

Thanks, Al. Everything you said makes sense. It's interesting that you took Dr. Johnson's course. It looks very interesting, although most of it would go way over my head.

Let me take this opportunity to say that, during this process, you've been an invaluable source of information, advice, and support. It is VERY appreciated!

I will now resume the rest of my life. My wife is expecting our first baby any day now, so I finished this experiment just in the nick of time! :-)

Bryon

I've been putting the finishing touches on reducing the effects of EMI/RFI by putting ferrites on things.

I've put some ferrites on power cords in the system (only on components whose performance is not affected by current draw).

I've also been wandering around the house, putting ferrites on things that dump RFI onto the power lines.

Question: Would it be effective, and is it safe, to put a ferrite on the GROUND WIRE of the system's dedicated AC line?

Bryon

Would it be effective, and is it safe, to put a ferrite on the GROUND WIRE of the system's dedicated AC line?

Hi Bryon,

I don't see any problem with that, although I have no idea as to whether or not it will provide a benefit. As you no doubt realize, what it will do is to raise the degree to which the the flow of currents that are at and above some frequency in the RF region is resisted. That should be no problem either under normal circumstances or under fault conditions that would require the breaker to trip.

Best,
-- Al

Thanks, Al.