Meaning
Characteristic opposition to alternating current in a transmission line where the signal conductor and return paths are situated on the same dielectric surface. Total coplanar waveguide impedance depends on the ratio of the center trace width to the distance between the grounded planes.
Geometric Influence
Spacing between the central strip and the lateral grounds provides the primary control over the electrical properties of the circuit. Narrow gaps increase capacitance and lower the characteristic value of the line. Wider gaps reduce the coupling and allow for higher impedance levels in the layout.
Substrate Interaction
Dielectric constant of the base material alters the effective permittivity seen by the propagating wave. This material property shifts the velocity of propagation and the physical dimensions required to reach a specific target. Glass reinforced laminates often present varying results based on resin content.
The height of the substrate also contributes when a bottom ground plane is present, turning the structure into a grounded coplanar waveguide.
Manufacturing Variance
Etching tolerances during PCB fabrication lead to small deviations in the copper widths and spacing. A change of one mil in the gap size typically results in a definite shift in the final coplanar waveguide impedance value.
