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.