Reducing Induction Effects, Counter-EMF, and Stray Voltages in Low-Voltage Communication Wires.

One of the observations which amaze older people like me, is how high the frequencies have become, at which even household appliances such as USB Cables can communicate. In my youth and young adulthood, such things would not have been considered possible. And the surprise which this progress brings, comes more strongly to older people, who actually did know about Electronics.

There is a basic enemy to allowing communication at high speeds: Plain wire has linear inductance, which becomes significant at the higher speeds.

There is a basic methodology to reducing the unwanted effect: Actual signal-wires are often accompanied by a shield wire, which needs to be grounded or connected to zero, at both ends of a wire bundle.

The concept is quite simple. This shield wire acts as a kind of secondary winding, to a virtual transformer, of which the signal wire would be the primary winding. Whatever counter-EMF the signal wire would produce, would also need to exist along the length of the shield wire. But because the shield wire is grounded at both ends, the counter-EMF which the signal wire can produce is also greatly reduced, in comparison with what one would obtain, if the signal wire existed by itself. When current flows in one direction through the signal-wire, current also flows in the opposite direction through the shield wire. If that current could not flow, then the full linear inductance of the signal wire would seem to exist. Otherwise, not so.

Now I suppose that it would be nice if overhead wires that stretch geographical distances, could also be shielded as easily. But one fact which is highly disappointing is, that shielding / elimination of stray-power problems, is highly lacking in many practical situations. More specifically, power lines may often only seem to have real phase wires, but no neutral wire that runs parallel. Instead, what some Engineers do, is simply to sink a grounding electrode into the earth, at the receiving end of such an arrangement.

The problem with that is the fact, that Earth is not a perfect conductor, and was also never ‘meant to’ participate in Humans’ high-voltage circuitry.

(Updated 3/12/2019, 15h20 … )

(As of 3/11/2019:)

OTOH, It makes slightly more sense in the case of overhead power-lines – meaning, distribution lines – to provide a neutral wire, but to ground that neutral wire. If that last step was not performed, then I suppose that a logical question which an Electrical Engineer would have to answer next would be, ‘What are we supposed to do with the neutral wire, even if we were only to connect the load to the relative voltage that exists between the phase wire, and the neutral wire?’ The answer to that question is actually different in different countries of the world.

In Germany, the neutral wire and ground are two different things, and if the Engineer was to connect them together, he’d actually be building a ground fault.

In Canada and the USA, the neutral wire is routinely tied to ground.


 

But my recent thinking has been more on the subject of those DSL wires, that carry higher frequencies for shorter distances. One of the main differences between how 10mbps and 50mbps DSL work is, that while 10mbps DSL still runs the two signal wires with constantly-opposing current, the 50mbps DSL actually sends a separate channel of bits through each of the two wires that supposedly form a pair. This technique is also known as “Pair Bonding”.

It should come as no surprise then, that common-mode voltage swings can disrupt the ability of a DSL modem to recognize bits belonging to the 50mbps service more, than they would disrupt the ability to function in 10mbps mode.


 

Also, when working with shorter, higher-frequency wire bundles, it can be said that an entire bundle of wires exhibits a phenomenon known as “Mutual Linear Inductance”, the same way single wires do. And then, actually to put a ferrite bead around a pair of wires, creates a special type of transformer called a “Balun Transformer”. This is an arrangement in which the mutual inductance has been maximized, so that one of the wires can be used as a primary, and the other as a secondary, in a pronounced way. The currents are effectively being forced to oppose each other.

 


 

(Update 3/12/2019, 15h20 : )

There exists a related phenomenon, which describes wire-pairs as high-frequency transmission lines, in which a voltage-swing is applied differentially to one end of the transmission line. The behaviour is, that this voltage-change propagates down the length of the transmission line at some fraction of the speed of light, and arrives at the other end.

This is usually the first phenomenon related to transmission lines, which people are taught. It’s a product of the linear inductance of the individual wires, acting together with the linear capacitance between them. The existence of this phenomenon is one of several reasons why, even if the shield wire I wrote about above is connected to zero at both ends, the voltage of the signal wire may also not be exactly equal to zero.

This differential mode of the transmission line is exploited at frequencies that go all the way up to microwave frequencies, and if the transmission line is well-designed, it will exhibit near-constant signal impedance, as well as near-constant propagation speed, over the desired range of frequencies.

Dirk

 

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