Impact of the common mode on high data rate transmission over balanced cabling

Inhalt:
It is common use to model the transmission of data over twisted pairs of conductors, in terms of the sole balanced mode, whereby one assumes that the pairs’ balance is perfect. Although this is never the case stricto sensu, this approach provided for accurate performance predictions, as long as the data rate was low. As the latter gets higher and 40 Gb/s is becoming a standard, the influence of the common mode increases and may not be neglected any more. It may be accounted for, based on “multi-mode” measurements of cables and connectors, taken separately. As demonstrated in an example, this new modeling method allows the engineer to predict how much the common mode can hinder balanced transmission. It is applicable to concatenated cables and connectors, with any number of pairs.