Differential signaling

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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:

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