Printed Circuit Board Design and Layout Questions

2. How do differential signals induce common-mode currents on the cables and structures attached to a printed circuit board?

As a signal propagates along a circuit board trace or transmission line, we normally assume that the current flowing away from the source is equal and opposite to the current flowing toward the source. For the purposes of signal integrity analysis, this is generally a good assumption. However, in practical electronic systems, there is always a small percentage of "common-mode" current that flows in the same direction on both signal conductors.

Definition of Common-Mode and Differential-Mode
See [1]

Basic mechanisms by which differential mode signals in a PCB induce common-mode currents on attached cables.
See [2-7]

Relationship between "balance" and common-mode currents
See [8, 9]

References

[1] Clayton R. Paul, Introduction to Electromagnetic Compatibility, 2nd ed., Wiley Interscience, 2006, pp. 348-349.

[2] D. M. Hockanson, J. L. Drewniak, T. H. Hubing, T. P. Van Doren, F. Sha, and M. Wilhelm, "Investigation of fundamental EMI source mechanisms driving common-mode radiation from printed circuit boards with attached cables," IEEE Transactions on Electromagnetic Compatibility, vol. 38, no. 4, Nov. 1996, pp. 557-566.

[3] T. H. Hubing and J. F. Kauffman, "Modeling the electromagnetic radiation from electrically small table-top products," IEEE Transactions on Electromagnetic Compatibility, vol. 31, no. 1, Feb. 1989, pp. 74-84.

[4] T. Hubing, "Printed Circuit Board EMI Source Mechanisms," Proceedings of the 2003 IEEE International Symposium on Electromagnetic Compatibility, Boston, MA, August 2003, pp. 1-3.

[5] D. M. Hockanson, J.L. Drewniak, R. E. DuBroff, T. H. Hubing, and T. P. Van Doren, "Consideration of Magnetic-Field Coupling Resulting in Radiated EMI," Proceedings of the 1998 IEEE International Symposium on Electromagnetic Compatibility, Denver, CO, August 1998, pp. 808-813.

[6] D. M. Hockanson, J. L. Drewniak, T. H. Hubing, T. P. Van Doren, and C. W. Lam, "Experimental and Numerical Investigations of Fundamental Radiation Mechanisms in PCB Designs with Attached Cables," Proceedings of the 1996 IEEE International Symposium on Electromagnetic Compatibility, Santa Clara, CA, August 1996, pp. 305-310.

[7] J. L. Drewniak, F. Sha, T. H. Hubing, T. P. Van Doren, and J. Shaw, "Diagnosing and Modeling Common-Mode Radiation from Printed Circuit Boards with Attached Cables," Proceedings of the 1995 IEEE International Symposium on Electromagnetic Compatibility, Atlanta, GA, August 1995, pp. 465-470.

[8] J. L. Drewniak, T. H. Hubing, and T. P. Van Doren, "Investigation of Fundamental Mechanisms of Common-Mode Radiation from Printed Circuit Boards with Attached Cables," Proceedings of the 1994 IEEE International Symposium on Electromagnetic Compatibility, Chicago, IL, August 1994, pp. 110-115.

[9] T. Watanabe, O. Wada, A. Namba, K. Fujimori, S. Matsunaga, R. Koga, "Equivalence of two calculation methods for common-mode excitation on a printed circuit board with narrow ground plane," Proceedings of the 2003 IEEE International Symposium on Electromagnetic Compatibility, Boston, MA, USA, Aug. 2003, pp. 28-33.