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PostPosted: Fri Jun 29, 2018 8:57 am 
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Joined: Fri Aug 16, 2013 8:23 am
Posts: 2345
Location: Seattle, WA, US
So here is a real test to see if the JSSG (John Swenson Shield Ground) really works - or is it just theory? ... es/?page=9
The important aspect here is the part "electrons in the metal move". What are moving electrons called? Current. In order for current to flow there has to be a loop. Current will not flow unless there is a loop. In the case of the Faraday cage, the loop is the whole cage. Lets say an electric field impinges on a side of the box, this causes a current flow that goes all the way around the box back to where the field impinges on the box. If the sides are not electrically connected to each other the current cannot flow, thus the electrons cannot move, and the canceling electric field does not get setup, thus no shielding.

It turns out that for AC electric fields it is a little different. Some current can flow due to capacitances between conductors. The electrons can move a little bit one way, then a little bit back. If the frequency is high enough the back and forth movement, which is charging and discharging the capacitance is sufficient for shielding. For a given capacitance the lower the frequency the less effective the shielding. The electrons start moving which charges the capacitance, then stop moving when fully charged, they don't do the full movement necessary to produce the canceling field.

So what about shielded cables? I hope is now obvious that for shielding to be effective there needs to be a conductive path from one end of the shield to the other. If there is not such a path the only shielding that is going to happen is for high frequencies due to cpacitances involved with the shield.

The best way for the shielding to work properly is a separate wire connected to each end of the shield. This is sufficient for shielding from DC to very high frequencies. Note the shield does NOT have to be connected an earth ground, the "ground" of the circuit at either end, or any thing else for that matter. A cable with a shield the is not connected to anything else except itself (ie a separate wire from one end to the other of the shield) will be highly effective in shielding what is inside.

Where does this wire need to go? It can be either inside or outside the shield, but if it is inside it can couple to the signal wires inside, so it is usually best to have it outside the shield. Note it has to be insulated from the shield except for the ends where it connects to the shield. It should intersect as little of the external field as possible so it should NOT be tightly spiraled around the cable. Just running along side the shield is best, although a very loose spiral (say one turn per foot) is almost as good.

So some ramifications of this: The traditional "connect the shield to one end and let the other end float" is not good, it does not allow a loop so shielding does not happen very well. If you add the external wire connected to the shield at both ends, then you CAN connect one or both sides of the shield to the signal ground or some other ground, but you don't NEED to for effective shielding. You will find that in many cases leaving the shield completely disconnected from the rest of the circuit is the best way to go, you get the benefit of properly working shielding without any interaction of the shield with your system. You may wind wind up with static charges on the shielding so a resistance from the shield to ground may be useful in some cases in order to dissipate static charges.

So how come nobody does this? I don't know. My only guess is that cable shielding has been going on long before the actual mechanism for shielding was worked out, thus by the time it was understood, cable shielding was "standard" and nobody ever even thought about analyzing it based on an understanding of how shielding actually works.

But shouldn't the big companies know about this? It seems they don't. I have read several app notes from Belden that state that shielding is only effective at high frequencies, at audio frequencies and power supply frequencies (60Hz etc) it is totally ineffective.

Audio people are the only ones that seems to at least empirically know about this. Remember phono cartridges and preamps, there is a little green wire that goes from the "ground jack" on the preamp to the tonearm. Everybody assumes that this is to "ground the cartridge" but what it really does is provide a loop from one end of the interconnect shield to the other, it has nothing to do with whether it is "grounded" or not. So if you have (or had) a turntable you were actually taking advantage of this without realizing it.

So there you have it, shielding DOES work, but only if you provide a path from one end of the shield to the other. This is effective even if you don't connect the shield to anything else.

John S.
Using a very sensitive ELF EMI detector - one with sensitivity down to 0.1mG (+/-5%) and measures in the ELF EMI range (30Hz-300Hz) where the JSSG is supposed to be extremely effective. And as stated above JS claims that having no JSSG loop on a shield makes it almost totally ineffective.

Here is the tester - note the detection sensor is at the top only:
s-l1600 (12).jpg
s-l1600 (12).jpg [ 45.39 KiB | Viewed 3002 times ]
So the whole tester does not have to be shielded.

Here we go - using a DELL SMPS as the ELF EMI emitter - here is the baseline measurement:
JSSG baseline.jpg
JSSG baseline.jpg [ 397.11 KiB | Viewed 3002 times ]

486mG pretty bad.

Now I placed the tester inside some 1 1/2 inch tinned copper braided shield NO JSSG, NO Earth Ground - And Wow!
Shield no JSSG.jpg
Shield no JSSG.jpg [ 498.4 KiB | Viewed 3000 times ]

Nice! 143mG - a 71% drop in this ELF (30Hz-300Hz) EMI noise

So much for the JS theory that normal copper braid shielding is ineffective in blocking ELF EMI.

"shielding DOES work, but only if you provide a path from one end of the shield to the other." Uh-huh...sure thing - except it does work - works amazing. :roll:

So what about the JSSG treatment?
Well it's very slightly effective - not even worth the bother. :roll: :roll: :roll:
JSSG.jpg [ 191.72 KiB | Viewed 3000 times ]

Well the reading dropped to 138mG - only increased the shield by less the 1% - from 71% to 72%!
So it seems all the Hyperblovating on CA about this JSSG or JSSG360 is a silly waist of time. :lol: :lol: :lol:

I knew it! :mrgreen:

What a bunch of BS tail chasing. Now that is settled - time to focus on real improvements that work!


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PostPosted: Tue Jul 10, 2018 7:07 am 
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Joined: Fri Aug 16, 2013 8:23 am
Posts: 2345
Location: Seattle, WA, US
JSSG 360 Test Results
I found some more large tinned copper braid shielding - so I wanted to try a double layer (without the JSSG360) and with so here are the results.

The original test without shielding produced a ELF EMI reading of 486 mG

Then with a 'short piece' of copper braid shielding 143mG a 71% reduction

Finally with the JSSG 'treatment on this shield 138mG only a 1% additional reduction

So with a longer stretch of shielding the results were better at 100mG - this I believe is due to a lack of a large opening on the top of the shield in the short example - so a more complete coverage of the sensor. We are now at a 79% reduction
Long Shield single.JPG
Long Shield single.JPG [ 471.15 KiB | Viewed 2651 times ]

So what about just adding in a second layer of shielding - No JSSG, JSSG360, or grounding - just a floating dual layer
Long Shield double No JSSG 360.jpg
Long Shield double No JSSG 360.jpg [ 340.16 KiB | Viewed 2651 times ]

Surprise another improvement - well not to surprising. Now at 48mG - a 90% reduction.

Well what about the JSSG on the single long shield?
Long Shield Single JSSG.jpg
Long Shield Single JSSG.jpg [ 316.84 KiB | Viewed 2651 times ]

Again only a very small additional reduction in ELF EMI from 100mG to 96mG.

So what about JSSG 360 - I have the two layers of shield separated by some thick shrink wrap. I taped the two ends tightly to form a connection at the top and bottom of the layers.
And the result?
Double JSSG 360 Shield.jpg
Double JSSG 360 Shield.jpg [ 280.11 KiB | Viewed 2651 times ]

Well it kinda works - marginally a reading of 41mG a reduction from a non-JSSG360 double shield layer of only an additional 2% to 92%.

Again not really a big deal - shielding yes! JSSG and JSSG360 not so much.


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