XT (Crosstalk)

In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit or channel to another.

Crosstalk is a significant issue in structured cabling, audio electronics, integrated circuit design, wireless communication and other communications systems.

Cabling Crosstalk

In structured cabling, crosstalk refers to electromagnetic interference from one unshielded twisted pair to another twisted pair, normally running in parallel. Signals traveling through adjacent pairs of wire interfere with each other. The pair causing the interference is called the “disturbing pair,” while the pair experiencing the interference is the “disturbed pair”.

Near end crosstalk (NEXT)
NEXT is a measure of the ability of a cable to reject crosstalk, so the higher the NEXT value, the greater the rejection of crosstalk at the local connection. It is referred to as Near End because the interference between the two signals in the cable is measured at the same end of the cable as the interfering transmitter. The NEXT value for a given cable type is generally expressed in decibels per feet or decibels per 1000 feet and varies with the frequency of transmission. General specifications for cabling (such as CAT 5) usually include the minimum NEXT values.

Power sum near end crosstalk (PSNEXT)
PSNEXT is a NEXT measurement which includes the sum of crosstalk contributions from all adjacent pairs as an algebraic sum of the NEXT of the three wire pairs as they affect the fourth pair in a four-pair cable (e.g., Category 6 cable). The Superior Modular Products White paper states that the testing process for PSNEXT consists of measuring all pair-to-pair crosstalk combinations and then summing all of the values for each pair. The specification was developed to directly address the effect of transmissions on multiple adjacent pairs on the pair being tested and is relevant to all connecting hardware and associated communications cables.
Cabling bandwidths in excess of 100 MHz (Category 5 cable bandwidth) make consideration of PSNEXT more important as Gigabit Ethernet through Cat-6 uses all four wire pairs simultaneously and bidirectionally. The additional wire pair usage and growing bandwidth increases the need to keep NEXT in check.

Far end crosstalk (FEXT)
FEXT measures the interference between two pairs of a cable measured at the far end of the cable with respect to the interfering transmitter.

Equal level far end crosstalk (ELFEXT)
ELFEXT measures the FEXT with attenuation compensation.

Alien crosstalk (AXT)
AXT is interference caused by other cables routed close to the cable of interest as opposed to signals contained in the same cable.

Related Articles