Shielded vs. unshielded Ethernet and Grounding

[jabbr]

1 hour ago, Fitzcaraldo215 said:

So, have you published those scope traces, even photos of them, as well as details of construction and connection details? If not, why let an as yet unaccepted bet for a measly $10k stand in your way?  The rest of us are all dying to know.

 

@mansr got it right: a nasty ground loop that runs through the shield connected at both ends... do you really need to see a scope trace? 

 

There's a spectrum of noise currents which share a ground return (loop). Ground loops should be uncontroversial. Anyone who's done enough connections has encountered them. They are well discussed. 

 

So-called "leakage currents" are really the same thing as ground loops but frequently flow through parasitic capacitance. Depending on the degree of parasitic capacitance as well as frequency, the leakage currents will have higher or lower magnitude based on well known equations -- the complex form of Ohm.s Law V = I.Z

 

In the case of Ethernet cables as we are discussing here, leakage currents can flow either through the shield (aka ground loops) or through the higher impedance of the PHY (parasitic capacitance of the ethernet transformer).

 

 

1 hour ago, Fitzcaraldo215 said:

 

But, if you are going to publish, could you also possibly do simple frequency response both ways at the output of the DAC?  It might be interesting to determine if, as mansr suggests, it might be a simple grounding issue with spikes at 50/60 Hz and harmonics thereof.  

 

This electronics is all well known and I'm not going to publish oscilloscope traces the "prove" Ohm's Law. Do you really want to pay me $1000/hr to prove this? You don't need me to... it is a simple ground loop/leakage current issue at the very least.

 

Same thing for USB ... break the ground at perhaps 90% of the issue goes away. That should be the take home message of this thread.

[mansr]

18 minutes ago, jabbr said:

So-called "leakage currents" are really the same thing as ground loops but frequently flow through parasitic capacitance. Depending on the degree of parasitic capacitance as well as frequency, the leakage currents will have higher or lower magnitude based on well known equations -- the complex form of Ohm.s Law V = I.Z

Right, so if the capacitance is small, only very high frequencies will get through. Noise in the megahertz region is inaudible by itself. However, if it is modulated at a low frequency, it might cause audible disturbances. That's still a lot easier to mitigate than noise entering through a direct connection, ground or otherwise.

[asdf1000]

22 minutes ago, jabbr said:

leakage currents can flow either through the shield (aka ground loops) or through the higher impedance of the PHY (parasitic capacitance of the ethernet transformer).

 

Hi jabbr. John Swenson has stated that he's had to build very customised testing gear to show the latter. Does your testing gear show the same or are just going off what John has said?

 

I have a lot of respect for John S btw, but just like to get an idea if anyone else has the means to easily see these things themselves.

[jabbr]

35 minutes ago, mansr said:

Right, so if the capacitance is small, only very high frequencies will get through. Noise in the megahertz region is inaudible by itself. However, if it is modulated at a low frequency, it might cause audible disturbances. That's still a lot easier to mitigate than noise entering through a direct connection, ground or otherwise.

 

Certainly. I'm not trying to be cute. I believe that most cases where cables make a difference have to do with grounding and noise and antenna effects and shielding. Like lifting the USB ground ... common theme.

[jabbr]

1 hour ago, Em2016 said:

 

Hi jabbr. John Swenson has stated that he's had to build very customised testing gear to show the latter. Does your testing gear show the same or are just going off what John has said?

 

I've asked what voltage levels the leakage noise under discussion are, or the currents ... if we are measuring picovolts or femtoamps, yes you need very specialized equipment (for example) ... but does this matter?

 

Quote

I have a lot of respect for John S btw, but just like to get an idea if anyone else has the means to easily see these things themselves.

 

Certainly measurement in the microvolt range is frequently done (active high impedance probes) [1], nanovolt measurements are more difficult etc. Lets not get the idea that you can simply fire up your 'scope and stick in a probe ... you can show that your cables can be effective antennas 

 

1. http://download.tek.com/document/48W_28061_0_HR.pdf

[asdf1000]

27 minutes ago, jabbr said:

but does this matter?

 

Well yes, I would assume you've verified everything you say? Not that I need to see any measurements but I would have assumed you've verified with your own eyes, for yourself?

 

I mean this respectfully of course - not a personal attack. I've followed many of your posts and you are clearly a clever guy.

 

Some of us (me!!) blindly follow what the clever people say but I would have thought you'd have verified this high impedance leakage stuff yourself.

 

Anyway I don't want to harp on too much. It's really just the topic of the high impedance leakage sailing through digital isolators and ethernet's little transformers that interests me.

 

[jabbr]

30 minutes ago, Em2016 said:

 

Well yes, I would assume you've verified everything you say? Not that I need to see any measurements but I would have assumed you've verified with your own eyes, for yourself?

 

I mean this respectfully of course - not a personal attack. I've followed many of your posts and you are clearly a clever guy.

 

Verified what? What I say about Ohm's Law? Probably had to verify that in Physics class some decades ago...

 

I'm asking a genuine question and not giving a specific answer but food for thought. Goes along with a common theme I've been posting about. Let me pose the full question.

 

What levels of leakage currents might be significant in audio circuits?

 

(I haven't verified the answer to this question with measurements).

 

microamps? what is the background current noise level in the circuit? If the current noise is in the milliamp range then microamp noise is unlikely to matter.

 

But for me personally: I get the best isolation transformer I can find (Topaz 0.0001 pF) to sit behind my audio system, and then remove cheap SMPS and use LPS when feasible even if to power a dc/dc converter. I also use fiberoptic ethernet ... so these reasonably inexpensive techniques mitigate leakage currents

 

so I'm being lazy and practical but if you want to be precise  you or anyone could measure the baseline noise of your circuit

rule of thumb:  say SNR 120dB, 4V full range = 4 microvolt noise floor

So these >1GigaOhm impedance leakage currents are rather likely below the noise floor of the circuit. (willing to look at specific measured numbers though).

 

 

 

 

[asdf1000]

7 minutes ago, jabbr said:

Verified what? What I say about Ohm's Law?

 

No, you stated: "leakage currents can flow either through the shield (aka ground loops) or through the higher impedance of the PHY (parasitic capacitance of the ethernet transformer)."

 

The part in bold is John's claim (which I believe of course). I was just wondering if you've verified this claim with measurements yourself or just assuming it's correct.

 

 

7 minutes ago, jabbr said:

What levels of leakage currents might be significant in audio circuits?

 

(I haven't verified the answer to this question with measurements).

 

Ok, no worries at all then.

 

[jabbr]

30 minutes ago, Em2016 said:

 

No, you stated: "leakage currents can flow either through the shield (aka ground loops) or through the higher impedance of the PHY (parasitic capacitance of the ethernet transformer)."

 

Oh that ... no I haven't measured what the parasitic capacitance of the Ethernet PHY.

 

I do know that leakage currents across copper Ethernet are/can be a concern in healthcare. 

 

Quote

 

The part in bold is John's claim (which I believe of course). I was just wondering if you've verified this claim or just assuming it's correct.

 

Not just a blind assumption ... I've read about it. Makes sense to me. The claim about >1 Gigaohm leakage currents (personally hate that terminology but equates to a nanoamp current) hasn't been demonstrated to me to be relevant e.g. above the noise floor (but willing to learn).

[mansr]

10 hours ago, Em2016 said:

No, you stated: "leakage currents can flow either through the shield (aka ground loops) or through the higher impedance of the PHY (parasitic capacitance of the ethernet transformer)."

As those are the only paths available, the statement is trivially true.

[asdf1000]

2 minutes ago, mansr said:

As those are the only paths available, the statement is trivially true.

 

Noted, but before John Swenson mentioned it, I'd never heard anyone talking about high impedance leakage currents sailing through ethernet transformers... 

 

Until then, only the shield path was really discussed.

 

So I was kind of asking if jabbr had actually seen what John S is talking about there... i.e. his own measurements.

 

But that was all cleared up above.

 

[mansr]

1 minute ago, Em2016 said:

Noted, but before John Swenson mentioned it, I'd never heard anyone talking about high impedance leakage currents sailing through ethernet transformers...

Probably because nobody else uses meaningless terms like "high impedance leakage current."

[asdf1000]

2 minutes ago, mansr said:

Probably because nobody else uses meaningless terms like "high impedance leakage current."

 

What was the old equivalent term/s before that?

[mansr]

10 minutes ago, Em2016 said:

What was the old equivalent term/s before that?

Terms for what?

[asdf1000]

36 minutes ago, mansr said:

Terms for what?

 

High impedance leakage currents.

 

I'm assuming if it was known before, it had another name?

 

[jabbr]

39 minutes ago, Em2016 said:

 

What was the old equivalent term/s before that?

 

Every electronic circuit has a schematic which has the major electronic components but also parasitic capacitances and inductances that although not listen on the formal schematic, are present in the actual circuit.

 

Leakage currents are currents that flow to the ground (chassis typically): https://www.sunpower-uk.com/glossary/what-is-leakage-current/

 

If you connected a wire directly from the signal to ground that would be a "short circuit" and a relatively large current would flow. That's because the wire has low impedance (this is all the complex form of Ohm's Law).

 

Leakage currents typically flow through parasitic capacitances (easiest way to think about this). The impedance of the path depends on the amount of capacitance. The lower the parasitic capacitance the higher the impedance. https://www.dummies.com/education/science/science-electronics/generalize-impedance-to-expand-ohms-law-to-capacitors-and-inductors/

 

Every circuit consists of current loops, typically in multiple directions, and when dealing with high frequencies these loops are three dimensional. The amount of current flowing in each loop is inversely proportional to the impedance of the particular loop. Higher impedance means lower current. 

 

I'm not trying to be clever here, just trying to demystify this. These concepts are essential to electronics, so as I've said, I haven't verified since Physics class (my formal training isn't EE though I've done some stuff). In cases where its important to measure leakage current to meet standards, there are leakage current meters. These do measure leakage currents through Ethernet among many other things and I've seen them used by my engineers and physicists a few years ago. Among the more difficult measurements we made were using single photon diamond detectors in the 1990s... probably off the shelf these days...

[jabbr]

Let me add that I believe @JohnSwenson used the term “high impedance leakage current” because he had to use a high impedance probe to measure low current leakages.

 

Certainly. If the probe is low impedance then the probe shunts away the current it is trying to measure, so that’s why you want an active high impedance probe for sensitive measurements — that’s its own topic

[mansr]

16 minutes ago, Em2016 said:

High impedance leakage currents.

 

I'm assuming if it was known before, it had another name?

That combination of words doesn't mean anything. Currents don't have impedances.

[Superdad]

1 hour ago, mansr said:

That combination of words doesn't mean anything. Currents don't have impedances.

 

As far as I can recall, John never used “current” in his reference to high-impedance leakage. Others have somehow chosen to make that part of the phrase. 

Of course it is measurable and we have published some examples.

[Jud]

16 minutes ago, Superdad said:

As far as I can recall, John never used “current” in his reference to high-impedance leakage.

 

I think I see a bunch of examples in the forum search.

 

Which doesn't much matter to me, but I'm not a technical person.  So I suppose anyone who wishes to be careful about using the technically correct terminology may want to foreswear the use of the term "current" following "high-impedance leakage" in future.

[jabbr]

2 hours ago, Superdad said:

 

As far as I can recall, John never used “current” in his reference to high-impedance leakage. Others have somehow chosen to make that part of the phrase. 

Of course it is measurable and we have published some examples.

Ha ha ... so high impedance leakage ... could it be voltage? Nope ... gas?!

[look&listen]

5 minutes ago, jabbr said:

Ha ha ... so high impedance leakage ... could it be voltage? Nope ... gas?!

Mockery make you seem less smart  

[Jud]

6 minutes ago, look&listen said:

Mockery make you seem less smart  

 

Not sure it was intended that way.  My take was that he was wondering how you would refer to it understandably to most folks without using the "c-word." But he can speak for himself.

[mansr]

1 minute ago, Jud said:

Not sure it was intended that way.  My take was that he was wondering how you would refer to it understandably to most folks without using the "c-word." But he can speak for himself.

First I'd like seeing it defined in a way that makes sense to most engineers. Talking about the impedance of a voltage or current is like asking what colour the speed of your car is.

[diecaster]

3 hours ago, Superdad said:

 

As far as I can recall, John never used “current” in his reference to high-impedance leakage. Others have somehow chosen to make that part of the phrase. 

Of course it is measurable and we have published some examples.

 

Yes, he did. All over the place. Look starting here and then throughout the thread:

 

 

If it is not "high-impedance leakage current", what is it?