What is a coplanar waveguide (CPW)?
A coplanar waveguide is a type of transmission line where the center conductor and ground planes are all on the same side of the substrate. It consists of a center strip conductor separated by narrow gaps from ground planes on either side. See also our PCB Impedance Calculator.
How does CPW differ from microstrip?
Unlike microstrip which has ground plane on the opposite side of the substrate, CPW has both signal and ground conductors on the same plane. This makes CPW easier to connect components and provides better isolation between adjacent lines.
What factors affect CPW characteristic impedance?
The characteristic impedance depends on the center conductor width, gap width, substrate thickness, and dielectric constant of the substrate material. Narrower gaps and wider conductors generally reduce impedance.
Why is effective dielectric constant important?
The effective dielectric constant determines the phase velocity and wavelength of signals in the CPW. It's typically between 1 and the substrate dielectric constant, as the field exists partly in air and partly in the substrate.
What is the typical impedance range for CPW?
CPW can achieve impedances from about 20Ω to over 200Ω depending on geometry. Common target impedances are 50Ω for single-ended signals and 100Ω for differential pairs.
How accurate are these CPW calculations?
These calculations use proven analytical formulas and are accurate for most practical CPW designs. For critical applications or unusual geometries, electromagnetic simulation may provide higher accuracy.
What substrate materials work well for CPW?
Common materials include FR-4 (εr≈4.4), Rogers materials (εr≈3.0-10.2), and PTFE-based substrates. Lower dielectric constant materials generally provide better high-frequency performance.