Twisted Pair Cabling in Communication

One of the major causes of interference is the cabling that comes out of the system. The long cables sometimes acts as antenna and create a spoil part in the EMI/EMC performance of the system. While 

shielding the cables is always a good choice and terminating the cable shield to the earth to eliminate any interference is a standard implementation. Shielded cables are not always used owing to the fact 

that they add additional cost. Either designers or customers always look for a low cost solution. To put it in designers perspective, the common mode currents through the cables, the susceptibility of the cables to external interference are some of the biggest headaches. While having shielded cables are always welcome, it is the responsibility of the designer to ensure proper filtering mechanisms are 

implemented in the design. 

One of the common types of cables preferred in case of unshielding are the twisted pair cables. The twisted pair cables helps to eliminate stray capacitance as there is balanced drive along the cable. The designer has to ensure that the wires are twisted form the source to the end to be more effective. More the number of twists lesser the effect of stray fields on the cable.  While the twisted pair helps to 

eliminate stray capacitance, it helps to eliminate undesired magnetic fields around the cables. Each cable when twisted causes magnetic field to be reversed and cancel out at each point. More the number of twists in a unit length, better the performance of the twisted pair cables. Co-axial cables are always a better choice than twisted pair as there is clear shielding mechanism in such cables. But if we consider economical factors, twisted pair is the preferred choice. Twisted pairs are of major use in the short distance high speed communication. USB cable uses twisted pair which helps reduce external noise and cross talk. While the power and ground are not twisted, the differential data line inside USB are twisted pair. Some USB manufacturers shield the twisted pairs with a shield separately which adds more immunity. Unshielded twisted pair (UTP) cabling is what the terminology used for these cabling. Other than USB, Ethernet and telephone wires also use twisted pair cables.