Shielded (STP) versus unshielded (UTP) twisted pairs: an EMC comparison

The respective pros and cons of shielded (STP) and unshielded (UTP) twisted pairs are reviewed. Besides their lower cost, the hidden advantages of UTP are a slightly better balancing and a simpler connector hardware. Their balancing and common mode (CM) rejection can even be improved by high quality differential transformers with integrated CM blocking chokes. However when it comes to immunity to severe RF fields and to kilovolt electrical fast transients (EFT), UTP cannot achieve the necessary decoupling for mast typical high speed bus applications. This paper calculates the various emission and immunity features of modern UTP and STP, for a quantitative comparison.