Posted by: Paul Kish on September 04, 2014
Two important channel transmission parameters that most are familiar with for Category 6A cabling to support 10GBase-T applications include alien crosstalk and insertion loss (IL).
Alien crosstalk is a measure of the noise coupling between adjacent channels that are in close proximity (i.e., sharing the same pathway). Insertion loss is a measure of the received signal and is dependent on the length of channel (i.e., the longer the channel, the weaker the signal).
But did you know that other unmentioned parameters related to the pair balance of cables and connectors are also good indicators of noise immunity and hence support for 10GBase-T?
These parameters are called mode conversion parameters, and until recently, they could not be measured in the field. But they now deserve more attention.
While alien crosstalk and insertion loss are equally important, mode conversion parameters are associated with the balance and relative noise immunity of the cabling system. They also relate to alien crosstalk coupling between channels, in particular for short distances.
The 8?pin modular plug is inherently unbalanced for the outer pairs (1?2 and 7?8) and pair (3?6) due to unequal capacitance between blades of the plug. This inherent unbalance of connecting hardware can contribute to excess alien crosstalk in short channels (e.g., 3 to 15 meters). This is especially important in the data center where the channel lengths can be quite short. For more information on this phenomena, see the Belden white paper “Short is Tough.”
Mode conversion parameters are described in TIA TSB-1197, a new document that was just recently approved for publication. TSB-1197 shows different noise coupling mechanisms that can contribute to additional alien near end crosstalk (ANEXT).
One source of additional ANEXT that is quite significant is due to near end mode conversion at connector interfaces. The mathematical equations are quite complex, but the concept is relatively simple. The additional source of ANEXT results from two mode conversions:
1. At the connector interface on the disturbing channel, from differential mode (DM) to common mode (CM).
2. At the connector interface on the disturbed channel, from CM back to DM.
To fully understand these mechanisms, you can refer to the figures that are included in TSB-1197. But the gist of mode conversion is that DM signals can be partly converted to CM and vice versa.
As illustrated in this graphic (Figure 12 from TSB-1197), mode conversion along the signal path from DM to CM on the disturbing channel, and then back again to a DM noise signal on the disturbed channel, can adversely impact transmission performance. And the worse the balance, the worse the impact.
The important point to realize here is that not all Category 6A connecting hardware is created equal.
Pair balance of a mated connection is affected by the compensation circuit elements and the routing of the traces on the PCB of the 8-position modular jacks. Manufacturers of connecting hardware therefore need to design their modular jacks to effectively compensate for the inherent unbalance of the modular plug for all pair combinations.
The enabling technologies built-in to the Belden 10GX connecting hardware ensures a well-balanced design with very low levels on mode conversion and superior ANEXT performance at high frequencies beyond 500 MHz.