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Mode conversion parameters

TIA TR 42.7 balance task group is making good progress in modeling the effects of mode conversion on alien crosstalk performance.

September 1, 2010  

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For this month’s column I wanted to zero in on mode conversion. Mode conversion parameters are relevant to understanding noise coupling mechanisms for balanced twisted-pair cabling (both unshielded and screened twisted pair cabling).

A task group in TIA TR 42.7 copper cabling systems subcommittee is currently developing a model to explain how mode conversion parameters can result in additional alien crosstalk between channels and additional noise from external sources.

Once the work is completed, the intent is to publish a Telecommunications Systems Bulletin that defines mode conversion parameters and determines the extent to which these parameters can affect the transmission performance of balanced twisted pair cabling.

Applications running over balanced twisted pair cables are designed to operate using differential mode (DM) signals.

Differential mode means that the applied voltage on each conductor of a pair has the same magnitude, but is opposite in polarity. For example, a 1 volt differential mode signal has +0.5 volts applied on one conductor of a pair and -0.5 volts applied on the other conductor of a pair, resulting in 1 volt across the pair. If the physical geometry of a pair is perfectly symmetrical the electromagnetic field generated by one conductor of a pair is effectively cancelled out by the electromagnetic field generated by the other conductor of a pair. In an ideal case for symmetrical pairs the induced noise coupling between pairs in the same cable (crosstalk) or between pairs in nearby cables (alien crosstalk) would be very small.

Mode conversion occurs when a differential mode signal is partly converted to a common mode (CM) signal because of capacitive and inductive unbalance to ground due to pair asymmetry.

Common mode means that the voltage on each conductor of a pair has the same magnitude and the same polarity. Let’s take the same example of 1 volt signal applied across a pair.

If the pair is imbalanced with respect to ground, the voltage on each conductor of a pair would be divided unequally, for example [+0.6V/-0.4V] across a pair. This voltage can be expressed as the sum of a common mode voltage (Vcm) of [+0.1V] superimposed on a differential mode voltage (Vdm) of [+0.5V/- 0.5V]. The mode conversion for this example is 20log(Vdm/Vcm) = 20log(1/0.1) = 20 dB.

A main source of mode conversion for balanced cabling is the mated plug/jack connection for an 8-pin modular connector.

The blades of an 8-pin modular plug are inherently unbalanced, in particular between the outer pairs (pins 1-2 and 7-8) and the split pair (pin 3-6), which can result in a significant amount of mode conversion especially at high frequencies.

Mode conversion occurs either within a pair or between pairs. Mode conversion within a pair is called Transverse Conversion Loss (TCL) at the near end or Transverse Conversion Transfer Loss (TCTL) at the far end. Mode conversion between pairs is called inter-modal near end crosstalk (NEXTcd or NEXTdc) or inter-modal far end crosstalk (FEXTcd or FEXTdc). Mode conversion between pairs is not a parameter that is currently specified in the TIA or ISO cabling standards.

So why study mode conversion parameters and why is it important? During the development of the Category 6A standard, it was determined that the power sum alien near end crosstalk (PSANEXT) coupling between channels was greater than predicted when adding up the PSANEXT between components (including cables, cords and connectors) using a differential mode crosstalk coupling model.

The amount of additional PSANEXT depended on the connector design and was worse for short channels with two or more connectors located in close proximity.

Some of the added PSANEXT was accounted for in the TIA 568-C.2 standard by including an allowance of up to 6 dB in PSANEXT requirements for short channels of up to 30 meters.

The TIA 568-C.2 standard also provides laboratory measurement procedures that can be used to validate that the cabling complies with Category 6A PSANEXT requirements for worst case short and long channel configurations.

The TIA TR 42.7 balance task group is making good progress in modeling the effects of mode conversion on alien crosstalk performance. The eventual publication of this TSB will provide a useful tool for cabling manufacturers to assess the performance of category 6A mode conversion parameters between pairs for cabling and components and for other common mode parameters.

We have come a long way as an industry in improving the performance of copper cabling to support new and more demanding applications.

Mode conversion parameters are a missing piece of the puzzle and are important to fully characterize cabling performance for the next generation cabling systems for 40 Gb/s and beyond. CNS

Paul Kish is Director, Systems and Standards at Belden. The information presented is the author’s view and is not official TIA correspondence.