Differential signaling

The voltage and current in even and odd mode:

The characteristic impedances in even and odd mode:

The common and differential impedances:

In general, a system of N longitudinal conductors hove N-1 TEM modes. So, two lines over a common ground plane have two TEM modes. The typical modes are even and odd mode.

Even mode - driven in phase with equal amplitudes

Odd mode - driven π out of phase with equal amplitudes

Note that the sign of voltage is relative to the static voltage, so that negative voltage indicates a transient from HIGH to LOW.

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If we define the voltage and current in the even and odd mode as

,then any modes can be expressed in terms of the even and odd mode.

The even and odd mode impedances are naturally defined as:

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In differential signaling, the differential voltage and current are

, then the differential impedance is double of odd mode imedance.

Similarly, the common voltage and current are

, then the common impedance is half of even mode impedance.

The impedances are depicted as follows.