Radiocommunications Agency  EMC Awareness

Balanced interconnections and twisted pair cabling

What this technique is used for

It is commonplace to transmit broadband data or sensitive signals some distance over a metallic (wired) connection. Typical examples would be local area networks or ISDN data lines. As the signals are carried along the cable they are exposed to external EM influences in the environment they pass through, and they may also couple into that environment and cause disturbances to other equipment in either the near or the far field. To minimize the extent of this coupling, the construction of the cable itself needs to be controlled.

How this technique is used

Twisted pair is a particularly effective and simple way of reducing interference coupling. Twisting the wires tends to ensure a uniform distribution of capacitances. Both capacitance to ground and to external sources are balanced. This means that common-mode capacitive coupling is also balanced, allowing high common-mode rejection of the cable system.

The crosstalk or external coupling to any cable, whether or not it is twisted pair, has both capacitive and inductive components. Twisting is also useful in reducing low-frequency magnetic pickup because it reduces the effective magnetic loop area to almost zero. Each half-twist reverses the direction of induction so that two successive half-twists cancel the wires’ interaction with the field. Effective loop pickup is now reduced to the small areas at each end of the pair, plus some residual interaction due to non-uniformity of the field and twist irregularity. If the field is localised along the cable, performance improves as the number of twists per unit length increases.

The equivalent circuit has the capacitive coupling as a current source onto each conductor half-twist while the inductive coupling is a voltage source in series with each conductor, with an opposite sign at each half-twist.

Key issues in employing this technique

Installation

The effectiveness of twisting a signal/return pair depends on the impedance and the balance or unbalance of the signal circuit.

For high impedance unbalanced circuits, capacitive coupling dominates and there is little reduction in overall coupling by twisting. As the circuit impedance drops so capacitive coupling reduces and the inductive part becomes dominant, so that twisting becomes progressively more beneficial. Twisting together power conductors (circuit impedances of a few ohms) is therefore good practice.

Balancing the circuit reduces the effect of capacitive coupling, since external interference is coupled equally to both halves of the circuit and is attenuated by its common mode rejection. Interaction with external fields is determined mostly by residual inductive coupling.

Longitudinal conversion

A common issue for equipment with data communication ports is how it will interface with cabling that has already been installed within a building, or "structured cabling". There is no clear preference as to whether the data cable should be shielded or unshielded. A good quality shielded cable can control interference coupling, but only if it is properly terminated and if the shield is unbroken along its length – not easy if it includes joints or patch panels. If this is not the case, then a well-specified UTP (unshielded twisted pair) cable is likely to be better overall, as long as the equipment to which it is connected has good common mode rejection.

If you are designing a product with a LAN or telecom data interface then you will need to decide which type of cable to use. If shielded, then you must provide for a correctly terminated shielded connector, and ensure that the installation uses this connector with the proper cable in the right way. If unshielded, then the connector is less important, but the interface must have good common mode rejection, which will mean ensuring that the physical layout is balanced and, usually, incorporating a wideband common mode choke. It will also be necessary to specify the Longitudinal Conversion Loss (LCL) of allowable connected cables – typically by restricting them to one or other of the IEC 11801 categories (Cat 3, 5 or 6).

The LCL of a balanced cable system describes the degree to which an inadequately balanced termination will develop an unwanted transverse (differential) signal when excited by a longitudinal (common mode) signal. It is measured as shown below. Although this diagram shows a differential mode signal generated by a common mode input, the principle is reciprocal and can be used to describe unwanted common mode signals developed by intended differential signals.