Description

A stripline is constructed with a flat conductor suspended between two ground planes. The conductor and ground planes are separated by a dielectric. The distance between the conductor and the planes is not the same for both reference planes. This structure will most likely be manufactured with the printed circuit board process.

Example

An example of an asymmetric stripline is a 4 layer pcb were a trace on layer 3 is referenced to both layer 1 and layer 4. The trace is closest to layer 4 and layer 4 has the dominant effect on the transmission line impedance, but layer 1 would still affect the characteristic impedance of this trace.

Asymmetric Stripline Transmission Line Models

The impedance Z_0,SS for an asymmetric stripline can be computed according to the formulas contained in the document IPC-2141A [1], in particular paragraph 4.2.5. The characteristic impedance is given by:

Eq. 1

where:

Z_0,SS is the symmetric stripline impedance computed according to Eq. 1 or Eq. 3 of the Symmetric Stripline Impedance tool , providing the following input values: Ɛ_r=1, b=h1+h2+t. Z_0,SS is the impedance with air as the dielectric and having total thickness, b, equal to h1+h2+t.

ΔZ_0,air is given by the following equation:

Eq. 2

Where h1 is the distance between the signal line and the lower reference plane, h2 is the distance between the signal line and the upper reference plane, and Z_0,air is given by:

Eq. 3

Z_0,SS is the symmetric stripline impedance computed according to Eq. 1 or Eq. 3 of the Symmetric Stripline Impedance tool , providing, in turn, the following input values:

Ɛ_r=1, b=h1. It is the impedance with air as the dielectric and having total thickness, b, equal to h1

Ɛ_r=1, b=h2. It is the impedance with air as the dielectric and having total thickness, b, equal to h2