CPW Structure
Coplanar waveguide (CPW) has the signal conductor and ground conductors all on the same surface of a substrate. Ground planes are placed in slots on each side of the signal conductor. This configuration allows direct connection of ground-signal-ground (GSG) probes for on-wafer RF measurements.
CPW vs Microstrip Comparison
| Property | CPW | Microstrip |
|---|---|---|
| Ground location | Same layer (coplanar slots) | Bottom of substrate |
| Probe compatibility | GSG probe directly on top | Requires special access |
| Dispersion | Low | Moderate |
| Radiation loss | Can radiate at higher modes | Lower (bottom-ground shields) |
| Circuit integration | Easy shunt elements (to coplanar ground) | Requires vias for shunt elements |
GCPW (Grounded CPW)
Grounded CPW (GCPW) adds a bottom ground plane to the standard CPW structure, preventing substrate mode excitation and reducing radiation at high frequencies. GCPW is the preferred choice for MMIC designs above 20 GHz and PCB designs requiring both GSG probe access and low dispersion.
GCPW effective permittivity (approximate): εe ≈ (εr + 1) / 2 for wide substrates (H >> slot gap) Slightly lower than pure microstrip for same εr
RF View: Load CPW or GCPW S-parameter files (.s2p from EM simulation or on-wafer VNA) for full analysis in RF View. Microstrip Calculator provides useful reference for phase velocity estimation. Free on Android.