What is crosstalk?

Crosstalk is an undesired phenomenon in electrical and electronic systems where a signal transmitted on a wire/cable/channel or circuit interferes and corrupts the signal on another neighboring wire/cable/channel or circuit and vice versa. It is an electromagnetic interference (EMI) issue that arises when the electromagnetic fields generated by one signal couple into another nearby signal path, causing interference (disturbance), noise, or a reduction in signal integrity. The EM signals that cause interference are referred to as the aggressors, while the EM signal affected by crosstalk is referred to as the victim. 

Figure: Crosstalk in PCB

Crosstalk is one of the key issues in communication systems, structured cabling, integrated circuit design, Printed circuit board (PCB) designs, and audio electronics.

Causes of crosstalk: 

The crosstalk is usually caused by undesirable capacitive (electric field), inductive (magnetic field), or conductive (i.e., through a shared conductive path) coupling from one wire/cable/channel or circuit to another nearby wire/cable/channel or circuit. Let’s discuss these coupling mechanisms.

Electromagnetic coupling or Inductive coupling (inductive crosstalk): When an alternating current or a time-varying signal flows through a conductor, it generates a time-varying magnetic field around that conductor. If there is a nearby parallel conductor/wire, this changing magnetic field can induce a voltage (i.e., interference voltage signal) in that conductor due to the principle of mutual inductance. This phenomenon is called inductive coupling, and the disturbance signal caused by this phenomenon is known as inductive crosstalk. 

Figure: Understanding Inductive coupling or inductive crosstalk

 The following equation represents the induced voltage (which causes the interference or crosstalk):

E = M × di/dt.

Where: 

E - Induced voltage in the nearby conductor.

M - Mutual inductance between the two conductors.

di/dt - Rate of change of current in the primary conductor.

Electrostatic coupling or capacitive coupling (capacitive crosstalk): When the time-varying electric field lines from one wire or conductor link with a nearby parallel conductor, wire, or circuit, it can induce an interference current signal i= C(dV/dt) due to the mutual capacitance C between the two wires or conductors (with air acting as a dielectric medium). This phenomenon is called capacitive coupling, and the disturbance signal caused by this phenomenon is known as capacitive crosstalk.

Figure: Understanding capacitive coupling or capacitive crosstalk

Conductive coupling (conductive crosstalk): This type occurs when interference is transmitted through a shared conductive path, such as a common ground or power supply.

Impact of crosstalk on communication systems, cabling, PCBs, ICs, and audio Electronics:

Crosstalk in communication:

Telecommunication or Telephony: Due to a crosstalk issue, a telephone conversation may become audible in another circuit. 

Data Transmission: Crosstalk can alter the shape of data signals and decrease the signal-to-noise ratio (SNR) of the original signal, causing data errors.

In wireless communication, crosstalk occurs as co-channel interference, degrading signal integrity.

Crosstalk in cabling: 

Crosstalk can occur between an unshielded twisted pair and another pair of wires. This phenomenon can impact the quality and reliability of communication systems and data transmission that use cables for signal transmission. There are several important terms related to crosstalk in cabling that are essential to understand including:

Near-End Crosstalk (NEXT): 

Figure: Understanding Near-End Crosstalk (NEXT)

NEXT is the measure of crosstalk (or interference) between two pairs of cables at the transmitter end (i.e., source end). It is a coax cable's performance parameter, expressed in decibels (dB), and it varies with the frequency of transmission. A higher dB value of NEXT indicates lower interference, which is desirable, as it means that less of the transmitted signal is coupling into neighboring pairs. Typically, the general specification of the cable (e.g., CAT 5) includes NEXT value (dB).

 Power Sum Near-End Crosstalk (PSNEXT): 

Figure: Understanding Power Sum Near-End Crosstalk (PSNEXT), interference from other cables affect second cable.

In a multi-pair cable (e.g., CAT 6 cable), PSNEXT is the measure of the combined effect of crosstalk (i.e., the sum of crosstalk) from multiple pairs to a single pair. It is a coax cable's performance parameter, expressed in decibels (dB), and it varies with the frequency of transmission. A higher PSNEXT value indicates lower cumulative interference, which is desirable for maintaining the quality of data transmission in communication. Typically, the general specification of the cable, such as CAT 6, includes the PSNEXT value (dB).

Far-End Crosstalk (FEXT): 

Figure: Understanding Far-End Crosstalk (FEXT)

FEXT is a performance parameter of coaxial cable and is the measure of crosstalk (or interference) between two pairs of cables at the "far end" of the cable (i.e., opposite of the transmitter end). It is expressed in decibels (dB), and it varies with the frequency of transmission. A higher FEXT value refers to lower interference, which is desirable. This value is typically specified in the datasheet of cables such as CAT 6 cables. The FXET value does not account for the attenuation of the signal due to signal travel over the length of the cable.

Power-Sum Far-End Crosstalk (PSFEXT): PSFEXT (dB) is similar to PSNEXT but is measured at the far end of the cable, opposite to the transmitter end. It is typically specified in the datasheet of multi-pair cables such as CAT 6 cables.  

Alien Crosstalk (AXT): AXT is the interference experienced by one cable (the victim) due to signals from other cables routed close to the cable of interest, typically in bundled or parallel configurations.

Figure: Alien Crosstalk (AXT)

Crosstalk in Printed Circuit Boards (PCB): 

Crosstalk can also occur in PCBs, where the signal in one trace interferes with nearby trace(s), resulting in poor performance of the PCB.

Crosstalk in ICs: 

Crosstalk can also affect the performance and reliability of integrated circuits (ICs).Understanding and managing crosstalk is essential in IC design to ensure reliable and high-performance electronic devices.

Crosstalk in Audio Electronics: 

In audio electronics, crosstalk refers to the unwanted transfer of audio signals from one channel to another. This interference can cause audio signals intended for one channel to be heard in another channel, leading to a degradation in audio quality and reducing channel separation.

Classification of crosstalk:

Types of crosstalk based on the direction of propagation: Forward crosstalk and backward crosstalk.

In the forward crosstalk, the crosstalk/interference signal propagates on the victim conductor in the same direction as the aggressor signal. The backward crosstalk propagates opposite direction to the aggressor signal. The aggressor signal is the signal in a conductor or line that induces crosstalk/interferes with the nearby conductor (i.e., the victim conductor). 

Crosstalk types based on the measurement site: 

Based on the measurement site, the crosstalk can be classified as Near-end crosstalk (NEXT) and Far-end crosstalk (FEXT).

Crosstalk types based on how it is quantified:

Power-sum NEXT (PS NEXT)

Power-sum FEXT (PSFEXT)

Power-sum Equal-Level crosstalk (PS-ELFEXT)

How can crosstalk be reduced or minimized? 

• There are several methods/techniques are used to minimize the crosstalk issue in electrical and electronic systems. Some of them are listed below.
• Increasing physical the distance between lines/conductors or reordering the wires
• Utilize shielded cables and shielded twisted pair cables 
• Reducing the length of parallel run sections
• Decreasing the distance between the traces and reference planes can decrease the crosstalk issue in PCB design.  
• Shielding the conductors or traces and components within PCBs to isolate sensitive areas.
• In PCB design, avoid adjacent signal layers (if possible).  If this cannot be avoided, route the signal traces on adjacent layers perpendicularly.
• Using a ground plane between two adjacent signal layers in PCB can reduce crosstalk
• Using differential signaling
• Converting an analog signal into a digital signal before transmission can significantly mitigate crosstalk
• Proper designing, such as impedance matching of lines and suppressing nonlinearities.