EMC Question of the Week: May 31, 2021
The far-end crosstalk (FEXT) between two single-ended signals routed on parallel stripline traces with matched terminations is generally
- much less than the near-end crosstalk
- much greater than the near-end crosstalk
- proportional to the trace separation
- proportional to the trace width
Answer
The best answer is “a.” For matched transmission lines in a homogeneous dielectric, the electric-field coupling and magnetic-field coupling cancel at the far end, but add in-phase at the near end. For this reason, the far-end crosstalk tends to be much lower than the near-end crosstalk. When the dielectric is not homogeneous (e.g., a pair of microstrip traces), the electric-field and magnetic-field coupling are still out of phase at the far end, but their magnitudes are not identical.
The crosstalk between two traces would never be proportional to their separation. Increasing the trace separation normally reduces crosstalk.
The crosstalk between two stripline traces is also unlikely to be proportional to trace width. In fact, it may increase or decrease with trace width depending on the relative position of the traces and their proximity to the planes.
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