Edge-coupled differential microstrip transmission lines are a common differential trace routing technique. There are four different types of impedance used to characterize differential trace impedance. This calculator can find both odd and even transmission line impedances. Modeling approximations can be used to understand the impedance of differential microstrip transmission lines.

Edge-coupled differential microstrip transmission lines consist of two traces referenced to the same reference plane. There is a dielectric material between them. There is also some coupling between the lines. This coupling is one of the characteristics of differential traces. It is generally a good practice to match differential trace lengths and keep the distance between traces consistent. Also avoid placing vias and other structures between these traces.

Differential Impedance Definitions

Differential Impedance: The impedance measured when two lines are driven with signals of opposite polarity. Zdiff is equal to twice the value of Zodd

Odd Impedance: The impedance measured when testing only one differential trace referenced to the ground plane. Differential signals need to be driven with signals of opposite polarity. Zodd is equal to half the value of Zdiff

Common Impedance is the impedance measured when two lines are driven with the same signal. Zcommon is half the value of Zeven

Even Impedance: The impedance measured when testing only one differential trace referenced to the ground plane. Differential signals need to be driven with the same signal. Zeven is twice the value of Zcommon