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:
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
Transmission Line Parameters
Pcb (printed-circuit board) Traces
Generic Building-Cabling Standards
100-Ohm Balanced Twisted-Pair Cabling
150-Ohm STP-A Cabling
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( )