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Differential and Common-Mode Velocity

In a configuration with a homogeneous dielectric (like a stripline ) the propagation velocities for the differential and common-mode (also read odd-mode and even-mode) signals are equal.

Configurations with inhomogeneous dielectrics (like microstrips), however, support slightly different propagation velocities for the two modes. The impact of this difference is not very great as long as there is not much coupling between the modes (see Section 6.8) and as long as the receiver remains sensitive only to the differential component of the signal. It is theoretically possible to observe deleterious effects from the difference in velocities under the following circumstances:

  • A differential signal is created.
  • Part of the signal is inadvertently converted to a common-mode signal.
  • The differential-mode and common-mode signals propagate independently, and with slightly different velocities, to the far end of a long transmission line.
  • The common-mode signal is inadvertently converted back to a differential-mode signal.

In this case the receiver will perceive a superposition of two incident waveforms with slightly different timing and amplitudes (the double-converted signal presumably being smaller).

POINT TO REMEMBER

  • Microstrips support slightly different propagation velocities for the differential and common modes. The impact of this difference is not very great.


Fundamentals

Transmission Line Parameters

Performance Regions

Frequency-Domain Modeling

Pcb (printed-circuit board) Traces

Differential Signaling

Generic Building-Cabling Standards

100-Ohm Balanced Twisted-Pair Cabling

150-Ohm STP-A Cabling

Coaxial Cabling

Fiber-Optic Cabling

Clock Distribution

Time-Domain Simulation Tools and Methods

Points to Remember

Appendix A. Building a Signal Integrity Department

Appendix B. Calculation of Loss Slope

Appendix C. Two-Port Analysis

Appendix D. Accuracy of Pi Model

Appendix E. erf( )

Notes





High-Speed Signal Propagation[c] Advanced Black Magic
High-Speed Signal Propagation[c] Advanced Black Magic
ISBN: 013084408X
EAN: N/A
Year: 2005
Pages: 163
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