What is Crosstalk?
Communication systems use analog signals for communication purposes and these signals tend to deteriorate while traveling through transmission medium because of attenuation, distortion, and noise. This leads to impairment in signals at the sender and receiver’s end.
Crosstalk is a type of noise signal that corrupts the actual signal while transmission through the communication medium. Crosstalk occurs when a signal transmitted on one cable interferes and corrupts the signal transmitted on another cable in close proximity.
It mainly occurs in communication systems involving copper wires for transmission such as UTP (unshielded twisted pair cable) or coaxial cable. It is one of the major issues in communication systems, audio electronics, and integrated circuit designs.
Crosstalk is mainly induced because of coupling between different signals transmitted using parallel adjacent cables. It is caused due to electrostatic or electromagnetic induction. Every analog signal has an associated magnetic or electric field that overlaps with fields of other signals moving in parallel thus creating a disturbance in other signals by inducing interference signals in other signals.
Types of Coupling in Crosstalk:
- Electrostatic coupling: It occurs when an electrostatic field generated by current flowing in a wire in close proximity with another parallel wire generates an interference signal in that parallel wire due to the virtual capacitance built between the two wires. It is also known as capacitive coupling and the interference signal generated by this phenomenon is known as capacitive crosstalk.
- Electromagnetic coupling: It occurs when a dynamically changing magnetic field is generated by alternating current flowing in a wire in close proximity with another parallel wire generates an interference signal in that parallel wire due to the mutual inductance between the two wires. It is also known as inductive coupling and the interference signal generated by this phenomenon is known as inductive crosstalk.
Classification of Crosstalk
Types of crosstalk of basis of the direction of propagation
- Forward Crosstalk: Forward crosstalk propagates in the same direction of the signal that induces crosstalk or creates a disturbance in the actual or the victim signal.
- Backward Crosstalk: Backward crosstalk propagates in the opposite direction of the signal that induces crosstalk or creates a disturbance in the actual or the victim signal.
Types of crosstalk on basis of site of measurement
1. Near-End Crosstalk (NEXT): Near-End Crosstalk refers to the disturbance in analog signal in one of the twisted pair cables due to the signal in another pair of twisted pair cables at the near end of the transmission medium i.e. near the source of data transmission. It occurs when outgoing data signal leaks and corrupts incoming data signal. As a result, the incoming signal gets mixed with the outgoing signal at the near end of the transmitting station.
2. Far-End Crosstalk (FEXT): Far End Crosstalk refers to the disturbance in analog signal in one of twisted pair cable due to the signal in other twisted pair cable at the far end of the transmission medium i.e. near the destination of data transmission. It occurs when incoming data signal leaks and corrupts outgoing data signal at the receiver end. As a result, the outgoing signal gets mixed with the incoming signal at the far end of the transmitting station.
Types of crosstalk based on how it is quantified
- Power-sum NEXT (PS NEXT): Power-sum NEXT refers to crosstalk measured in terms of absolute or relative power at the near end of the transmitting station.
- Power-sum FEXT (PSFEXT): Power-sum FEXT refers to crosstalk measured in terms of absolute or relative power at the far end of the transmitting station.
- Power-sum Equal-Level crosstalk (PS-ELFEXT): Power-sum ELFEXT refers to the sum of PSNEXT and PSFEXT.
Alien crosstalk refers to the signal interference between adjacent cable links. Generally, it occurs when a cable (known as victim cable) is surrounded by many other cables known as disturbers that corrupt the signal of the victim cable. This becomes significant when the bandwidth of cables is increased for faster applications. Shielded cables also fail to prevent such kind of crosstalk.
- Signal integrity issues
- Noise-on-delay effect
- Logic faults
- Voltage overshoot
- Timing noise
- Reduce the length of parallel wires carrying signals.
- Use voltage lines with an equal magnitude but opposite polarity to generate signals.
- Use twisted-pair cables with more turns per unit distance.
- Replace UTP cables (unshielded twisted pair cable) with STP cables (shielded twisted pair cable).
- Convert analog signals to digital signals before transmission through the medium.
- Increase the twisting frequency of twisted-pair cables.
- Increase the distance between adjacent parallel cables carrying analog signals.
Challenges and Solutions:
- Communication System: Crosstalk leads to inefficient communication which may result in the leak of information from one line to another. Signals transmitted in the form of current over copper wires generate electromagnetic interference signals in nearby wires and wires pick up the signals from a wire carrying the information.
Solution: An increase in the twisting frequency of wires reduces the crosstalk in the communication system significantly. Twisting operations in the cables carrying signals play a critical role in preventing electromagnetic interference between the wires.
- Networking: Crosstalk within a network leads to inefficient data transmission which may result in loss of information. Signals transmitted on copper cables interfere with signals transmitting on another cable resulting in a poor signal-to-noise ratio of the network.
Solution: Shielded Twisted Pair Cables (STP) should be used instead of Unshielded Twisted Pair Cables (UTP) to reduce crosstalk within the network.
- Printed Circuit Board: Crosstalk on Printed Circuit Board(PCB) leads to electromagnetic coupling between the traces of the board. An excessive voltage or current in one trace creates interference on another trace on the board due to coupling between the two.
Solution: Crosstalk on PCB can be reduced by increasing the distance between the traces on the board, using differential signals, or avoiding parallel traces on the board.
Crosstalk is one of the major drawbacks of using copper cables as a transmission medium in communication systems. Managing crosstalk efficiently increases the reliability of transmission and makes copper cables a strong candidate of the transmission medium to be used in long-distance communications.