Shielding components from EMI/RFI... Help please

Hi Al - I checked for continuity between the tabs in the computer's ethernet port and the computer's IEC ground pin. You were right, the port is grounded.

Correct me if I am wrong, Al, but that seems to help explain something that puzzled me at the time: the dramatic improvement I heard when I first replaced the unshielded ethernet cables with shielded ones. In my posts on 2/16, I was mystified about how a $7 cable could make such a difference. You offered me a speculative explanation at the time, but I was left with a certain amount of puzzlement. What I didn't know at the time (but you did) was that the shielded ethernet cable was also grounded. Knowing that now, it seems to help explain how I could have heard such a dramatic improvement.

There is one other point I've been wondering about with respect to ethernet grounding, and that is: Is it better to ground an ethernet cable at both ends? I poked around a little and I found a discussion of this topic, in which an evident expert said this...

In high-speed digital applications, a low impedance connection between the shield and the equipment chassis *at both ends* is required in order for the shield to do its job...

In high-speed applications involving low-impedance circuitry, most of the near-field energy surrounding the conductors is in the magnetic field mode, and for that problem, only a magnetic shield will work. That’s what the double-grounded shield provides. Grounding both ends of the shield permits high-frequency currents to circulate in the shield, which will counteract the currents flowing in the signal conductors. These counteracting currents create magnetic fields that cancel the magnetic fields emanating from the signal conductors, providing a magnetic shielding effect.

For the magnetic shield to operate properly, we must provide means for current to enter (or exit) at both ends of the cable. As a result, a low-impedance connection to the chassis, operative over the frequency range of our digital signals, is required that *both* ends of our shielded cable.

One thing I'm still unclear about is whether the grounding I've created at both ends of my 50' ethernet cable is "operative over the frequency range of [the] digital signals." I know you said that...

the inductance of a plain piece of wire will cause it to have a significant impedance at the high RF frequencies that constitute the spectral components of the ethernet signals.

...which, in combination with the information I quoted above, seems to suggest that grounding the ethernet cable at both ends should have no additional benefit. But I DID hear an additional benefit when grounding it at both ends. Perhaps that is because, as you said...

the resistance of the very long run presumably lessens the effectiveness of that ground from the perspective of the switch.

...but I'm not sure I completely understand how that works. In other words, I'm unclear about how the following 3 things can all be true at the same time:

1. The effectiveness of a ground method for an ethernet cable's shield depends upon whether the ground method operates over the frequency range of the ethernet signal.

2. The ground method I provided my ethernet cable (at the switch) is ineffective at the frequencies range of the ethernet signal.

3. I heard an improvement when grounding the ethernet cable at *both* ends.

Assuming I'm right about #3, maybe you can say a few more words about how those things can all be true. Thanks!

Bryon

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