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A Balancing Act

What is balanced cabling? The answer to the question is not as straightforward as it seems.


May 1, 2003  


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For this month’s article I wanted to talk about the subject of balance. Both the TIA and the International Standards committees refer to copper cabling for customer premises as balanced twisted-pair cabling or balanced cabling.

What do we mean when we use the term balanced cabling? The answer is not as straightforward as it seems. In fact there is considerable effort underway in the TIA TR 42.7 and TR 42.9 subcommittees to define and specify cabling “balance” performance for commercial and industrial environments.

So what is balance? The recent Addendum No. 1 to the TIA 568 B.2 standard for Category 6 cabling provides recommendations on balance of cabling components. It states: “Balance ensures that the appearance of undesired signal coupling modes is minimized and is related to the emission and immunity characteristics of the cabling.” For the layman that is not well versed with the terminology of signal coupling modes, this statement doesn’t help much. The international standard ISO/IEC IS 11801:2002(E) introduces a new term “Unbalance Attenuation” to describe balance. It states that it is the ratio between the unwanted common mode signal power and the injected differential mode signal power. This common mode signal, which arises from imperfections in the cabling system such as asymmetry, causes electromagnetic emission and affects noise immunity.

This definition provides more detail but also introduces new terms like common mode signal and differential mode signal that may not be familiar terms for some readers.

BACK TO BASICS

So let’s start with the basics and take a look at “balance” and see how it relates to the construction of twisted-pair of conductors in a 4-pair telecommunications cable. A well-balanced pair is made of two insulated conductors, where each conductor is concentric and where the diameter over the insulation and the diameter of each conductor are nearly identical.

Furthermore, each conductor of a pair is symmetrically and uniformly oriented with respect to the surrounding conductors along its length. In such an arrangement, each conductor of a pair would have the exactly same transmission characteristics as it corresponding mated conductor.

If we applied an equal and opposite signal on each conductor of a balanced pair (i.e., a differential mode signal), the electromagnetic field generated by one conductor would be cancelled out by the electromagnetic field generated from its mated conductor.

There would be no resultant current flow in the surrounding conductors of the cable or in the ground return path.

On the other hand, if the pair were unbalanced because of physical asymmetry, one conductor would generate a stronger electromagnetic field than its corresponding mated conductor, thus causing a current to flow in the surrounding conductors within the cable or in the ground return path.

This resulting current flow in the surrounding conductors of the cable or in the ground return path is called a common-mode signal.

TIA 568 B.2-1 (Category 6 Addendum) defines a new parameter for measuring the balance of twisted-pair cables and connecting hardware. The parameter is called Transverse Conversion Loss (TCL).

Balance is an important parameter of a twisted-pair cabling system as it determines the degree of external noise coupling into the telecommunications cable from outside noise sources, such as impulse noise from power cables and RF antennas that are in close proximity.

Now that we understand something about balance, we can see that it is a parameter that is intrinsic to the design and manufacture of high performance twisted pair cables and connecting hardware. Many of the design and process improvements that yield better Return Loss and Crosstalk performance also improve the balance of Category 6 components.

Tests performed in the TIA balance task group on a cross-section of cables from the industry show better balance (TCL / LCL) measurements for Category 6 cables compared to Category 5e. This result correlates well with the other noise immunity tests performed in the TIA Power Separation Task group.

The relative noise immunity for a 90-meter Category 6 link was two times better than Category 5e when tested under the same test conditions.

In summary, the recipe to achieve good balance performance is a precision design and a tightly controlled and uniform manufacturing process.

Paul Kish is Director, IBDN Systems & Standards at NORDX/CDT. He is also vice-chair of the TR-42 engineering committee.