AN 672: Transceiver Link Design Guidelines for High-Gbps Data Rate Transmission

ID 683624
Date 1/29/2020
Public Crosstalk Control

Crosstalk is induced noise current resulting from mutual capacitive (Cm) and mutual inductive (Lm) coupling on a victim trace due to switching activity from nearby aggressor trace or traces. The current coupled from Cm travels along the victim trace in both the forward and reverse direction with the same polarity. Similarly, the current from Lm travels forward and backwards in opposite polarity. As a result, crosstalk can be separated into two distinct components referred to as near-end crosstalk (NEXT) and far-end crosstalk (FEXT). In NEXT, the coupled noise current is the sum of the induced currents from Cm and Lm as those currents are the same polarity. Conversely, for FEXT, the current is the difference of Cm and Lm due to the polarity difference. For signals entirely contained within a homogeneous dielectric material (such as stripline), the capacitive and inductive forward crosstalk are equal and cancel. For non-homogeneous dielectrics (such as microstrip), the inductive component tends to be larger and the resulting coupled noise is negative.

Figure 17. NEXT and FEXT Coupling Components

Crosstalk control usually involves reducing signal edge rates and maintaining enough trace-to-trace separation to reduce the mutual capacitive and mutual inductive coupling energy. In high-speed transceiver designs running at many gigabits per second, reducing the signal edge rate is usually not an option since the unit interval time (UI) is very small. Therefore, crosstalk control for high-speed transceiver designs is mainly determined by PCB layout spacing constraints to keep the transceiver traces far enough apart to minimize the coupling effect. For very high-speed traces, it is desirable to keep the coupling noise to less than 1% of the source signal if possible.