Unbalanced cable "directionality"... mismatched instructions?!?!

[Steve Eddy]

Originally posted by lgo51
A current will flow in a conductor that experiences an [electro]magnetic field.

Well yeah, you can cause eddy currents to flow within the conductor. But that's limited to the conductor itself.

Consider your radio or TV antenna, no physical closed loop there but it does have currents determined by the wavelength(s) of the impinging EMF.

There needn't be a physical, i.e. DC path, closed loop (though most AM radios do use a closed loop antenna). The loop may also be AC. Put a capacitor across the output of a component. There's no DC path closed loop, but there's certainly an AC loop there.

They are induced in the conductor and flow to "ground". No different than what we're speaking about in regard to interconnect wires.

But it is different.

In the case of an antenna, half the antenna is connected to the circuit's input and then to the other half of the antenna through a load impedance, the other half of the antenna being tied to ground.

However in this case, we just have a wire that's open at one end and the other end terminates at the ground node.

Now according to Kirchhoff, the current flowing into that node must be the same as the current flowing out of it. So if you've got current flowing from the wire into the ground node, where is that current going in order for it to flow out of that node?

se

[lgo51]

Originally posted by Steve Eddy
Now according to Kirchhoff, the current flowing into that node must be the same as the current flowing out of it. So if you've got current flowing from the wire into the ground node, where is that current going in order for it to flow out of that node?

To "ground", maybe through the input stage of the device, maybe elsewhere. Point is that that junction is not "ground", regardless of its common label, and since there is current flow through the junction that "noise" now gets into the frontend simply because the signal reference has now "moved" and the Vin "seen" by the frontend has deviated from that which the signal-only would have presented. That's the theory as I understand it. And I'm willing, nay happy, to be shown the folly of my thoughts. Seems difficult to believe that this all to common issue has not been modeled by better minds and a definitive engineering perspective established. Maybe someone could point us to such a treatise?

Cheers,
LarryO

[Steve Eddy]

Originally posted by lgo51
To "ground", maybe through the input stage of the device, maybe elsewhere.

Um, it's tied to the ground node, not the input stage of the device.

Point is that that junction is not "ground", regardless of its common label...

If the ground node isn't ground, then what exactly do you consider ground to be?

...and since there is current flow through the junction that "noise" now gets into the frontend simply because the signal reference has now "moved" and the Vin "seen" by the frontend has deviated from that which the signal-only would have presented.

How has the signal reference moved? Moved relative to what?

That's the theory as I understand it. And I'm willing, nay happy, to be shown the folly of my thoughts.

That's a good thing. I'm not exactly saying you're wrong. But before I say one way or the other I need to understand exactly what it is you're trying to say.

Seems difficult to believe that this all to common issue has not been modeled by better minds and a definitive engineering perspective established. Maybe someone could point us to such a treatise?

I think the reason for the different recommendations has to do with those making the recommendations considering things other than the role of shielding and are considering things such as ground loop and other interchassis curents which have rather nothing to do with shielding.

From a purely shielding perspective, it seems to me that the shield sound be an extension of the chassis and connected at both ends. However this can bring about its own problems as mentioned above.

se

[AR2]

I am sure that these articles will help you. I find them very informative regarding ground woodoo.
http://www.rane.com/note110.html
http://www.rane.com/note151.html

AR2

[lgo51]

Originally posted by AR2
I am sure that these articles will help you. I find them very informative regarding ground woodoo.

Thanks AR2 for the pointers. Spent the better part of the [ahem... work]day bouncing down the hypertext link nexus from there. Learned many things, and read other interesting stuff. Here are some of the take aways...

Following up on the conversation with SteveE, the folks at Rane stated the issue with more clarity than i did....

Sound System Interconnection, Rane Technical Staff
"The potential or voltage which pushes these noise currents through the circuit is developed between the independent grounds of the two or more units in the system. The impedance of this circuit is low, and even though the voltage is low, the current is high, thanks to Mr. Ohm, without whose help we wouldn't have these problems. It would take a very high resolution ohm meter to measure the impedance of the steel chassis or the rack rails. We're talking thousandths of an ohm. So trying to measure this stuff won't necessarily help you. We just thought we'd warn you."

The lesson was that there is inductance and resistance in all the conductors we typically think of as "good conductors". Here's a snippet that puts some dimensions to that notion...

Design Techniques for EMC – Part 2: Cables and Connectors, By Eur Ing Keith Armstrong CEng MIEE MIEEE
"All cables suffer from intrinsic resistance, capacitance, and inductance. Forgetting fields and antennas for a moment: a few quick-and-dirty examples will show how even very tiny departures from the ideal cause problems for signals carried by conductors at commonplace modern frequencies.

· The resistance of a 1mm diameter wire at 160MHz is 50 times more than at DC, due to the skin effect forcing 67% of the current to flow in its outermost 5 microns at that frequency.

· A 25mm long 1mm diameter wire has an intrinsic space-charge capacitance of around 1pF, which does not sound much but loads it by around 1kW at 176MHz. If this 25mm long piece of wire alone was driven in free space by a perfect 5V peak-to-peak 16MHz square wave, the eleventh harmonic of the 16 MHz would take 0.45mA just to drive the wire.

· A connector pin 10mm long and 1mm diameter has an intrinsic inductance around 10nH, which does not sound like much. When driven with a perfect 16MHz square wave into a backplane bus impedance that draws 40mA, the voltage drop across this pin will be around 40mV, enough to cause significant problems for signal integrity and/or EMC.

· A 1 metre long wire has an intrinsic inductance of around 1mH, preventing surge protection devices from working properly when used to connect them a building’s earth-bonding network.

· A 100mm long earth wire for a filter has so much intrinsic inductance (around 100nH) that it can ruin filter performance at > 5MHz or so.

· The inductance of a 25mm long “pigtail” termination for the screen of a 4 metre cable is enough to ruin the cable’s screening effectiveness at >30MHz or so.

The rules of thumb for intrinsic capacitance and inductance for wires under 2mm diameter is 1pF per inch and 1nH per mm (sorry to mix units, but they stick in the mind better)."

Given these physical realities, how does one deal with them? Well that was an open question, especially in the pro-audio world, for many years. Over the years many 'common practice' approaches were adopted, most of with where based upon trial and error in any particular situation. And, as many attested, the results were often less than wonderfully effective.

What I discovered was that the fog surrounding this subject was effectively lifted by the work of many in the AES, such as Neil A. Muncy and Ralf Morrison. And shown to be effective by Tony Waldron and Keith Armstrong who performed some key experiments that fully supported the proposed techniques. The later was a great read, here's an excerpt...

Bonding Cable Shields at Both Ends to Reduce Noise, by Tony Waldron and Keith Armstrong
"The authors found plenty of anecdotes but few hard facts when investigating the effect of ground loops on signal noise, so performed some tests themselves and reached some very interesting and valuable conclusions. This article describes tests we performed on a variety of balanced audio cables nearly 30m long with metallised foil or braid shields, to determine the effects of power-frequency shield currents (ground loop currents) on noise. Several types of balanced audio cables were tested, including an extremely poor quality balanced audio cable with untwisted signal conductors and a capacitive imbalance exceeding 20%.

Analysis of the test results revealed, to the authors’ surprise, that power frequency currents in metallised foil or braid shields do not inductively couple significant noise into their internal conductors even at current levels which cause the cables to warm up. However, the voltage between the cable’s shield and its internal conductors is a significant source of noise for balanced signals which have high impedances to ground. For good quality pro-audio balanced cables and equipment, the signal noise created by the equipment’s common-mode rejection is comparable with that created by capacitive imbalance in the cables.

It appears that traditional equipment design methods that connect cable shields to conductors inside equipment are most probably to blame for the problems which have been blamed on ground loops. Equipment constructions that bond cable shields directly to the chassis/frame/enclosure are better for EMC compliance and allow signals to achieve the highest levels of quality regardless of the ground loop currents flowing in their cable shields."

From all of this I jotted down a list of guidelines...

1) Never connect a circuit's "signal ground" to the chassis.
2) Never connect the main's "earth" line to the circuit's "signal ground".
3) Always connect the mains "earth" line to the chassis.
4) If using a "Y Filter" on the mains lines ("hot", "neutral") for common mode noise supression, reference the legs to the mains "earth" line at the point it connects to the chassis, and mind rule (2).
5) All signal interconnects should be fully balanced. If this is not possible (legacy equipment, circuit constraints, etc.), then noise performance will be compromised.
6) The "shield" of an interconnect should be connected to the chassis at both ends of the cable. And, that connection is best if it completely envelops the cable/connector junction (so called 360 degree connection) without the use of "pigtails".
7) It is best if each chassis is additionally strapped to a ground reference point by a very low impedance (inductance + resistance) connector.
8) Internal shielding to segregate noisy/dirty circuits is a good idea.
9) Contrary to conventional thinking, a "meshed ground" is better than a "star ground". This is true whether doing circuit board layout or lightning protection systems.

AND, the following related notes were taken...

1) 2) Should one 'correct' legacy equipment?
3) There is a branch of equipment design, called "double insulated" that intentionally does not adhere to this practice and it would be a bad/unsafe idea to alter such designs without specific expertise.
4) These filters use special capacitors that always fail to an open-circuit condition so do not think "audiophile" for this application.
5) This is the big hitch in the getalong. There is a gaggle of legacy and, sad to say, current equipment that does not support this highly logical principle. There are a number of possible solutions that can be applied to this situation. Note 110 from Rane, cited above by AR2, has a nice discussion of this and a handy table. The issue of floating one end of the "shield" is delt with extensively. I could find no solid engineering to form a rule for which end (source or target) to bond the "shield" to and which end to 'float'. I can say that more often I noted that the shield was tied to the target (input circuit) end of the cable. There was much said about the effectiveness of using a small (1 - 100 nF) capacitor between the "shield" and the chassis/signal ground at the 'floating' end of the cable. This completes the circuit for RF frequencies and blocks the low frequency 'ground loop' currents. Seems like solid concept and often recommended, but a challenge to implement physically.
6) The "360 degree" vs. "pigtail" connection techniques have more significant affects above about 30 MHz so for audio gear (sans digital circuitry) this level of attention may have diminushed return.
7) Lots of talk about this, especally related to lightning safety and in situations where there is significant distance between component equipment and the chance of higher "ground potential" (remember, there's no real "ground") differences is greater. One highly interesting point was that copper foil ribbon (say 0.011 x 2.0") has lower impedance than a #6 copper wire, and is much easier to handle too (remember to avoid sharp bends). For good discussions on this topic see the ARRL and other HAM sites.
9) This was one of those ratholes that I spent some time reading on as it was so contrary to what I thought I knew. Bottom line is that it works, and for good reasons. Trouble is that I have not (yet) found a good reference on "how to mesh ground" for general circuit wiring/layout techniques. Star grounding on the other hand has been addressed many times and seems almost intuitive.

That's what I gleaned from my little oddesy into the world of grounding and shielding. Maybe some of the more learned here will correct the things I've misinterpreted or help answer some of the open questions.


Cheers,
LarryO

[lgo51]

Here are some articles I found to be informative...

Designing for Interference-free Audio System Components
Design Techniques for EMC cables and connectors
Bonding Cable Shields at Both Ends to Reduce Noise
A Practical Interference Free Audio System (Part 1)
A Practical Interference Free Audio System (Part 2)
Grounding and Shielding Audio Devices
Sound System Interconnection


LarryO

[jan.didden]

Larry,

You may also like to look at this:

http://www.analog.com/UploadedFiles/...8865AN-202.pdf

(ADI AN-202).

Not strictly about cabling and screening, but defitively On Topic.

Jan Didden

[AR2]

Originally posted by lgo51
Here are some articles I found to be informative...

Designing for Interference-free Audio System Components
Design Techniques for EMC cables and connectors
Bonding Cable Shields at Both Ends to Reduce Noise
A Practical Interference Free Audio System (Part 1)
A Practical Interference Free Audio System (Part 2)
Grounding and Shielding Audio Devices
Sound System Interconnection


LarryO

Hi LarryO,
Could you please post the link to this articles.

[lgo51]

Sorry, just have the docs. All should be a 1st pg hit on Google.