Coplanar Waveguide vs Microstrip
| Property | CPW/GCPW | Microstrip |
|---|---|---|
| Ground plane location | Same surface (slots either side of trace) | Bottom of substrate |
| Probe measurement | Easy (GSG probe on top surface) | Requires flip-chip or via |
| Dispersion | Low (air fills upper half) | Moderate (εe frequency-dependent) |
| Radiation loss | Moderate (mode coupling) | Lower (shielded by ground plane below) |
| PCB compatibility | Requires tight slot tolerance | Standard trace width tolerance |
Grounded CPW (GCPW) Impedance Formula
GCPW has: signal conductor (width W), ground slots (gap G), and bottom ground plane (substrate height H) Effective dielectric constant (approximate, W/H large): εe_CPW ≈ (εr + 1) / 2 Note: RF View's microstrip calculator does not directly compute CPW impedance. For CPW design: use AppCAD, Sonnet, or TXLine calculator first, then verify with RF View S-parameter analysis of measured/simulated CPW.
Using RF View for CPW S-Parameter Analysis
While RF View's Microstrip Calculator is designed for microstrip, you can use it for CPW by entering the CPW's effective parameters:
- Use an external CPW calculator (AppCAD, AWR TXLine) to find εe and Z₀ for your CPW geometry
- In RF View Microstrip Calculator: enter the computed εe (instead of substrate εr) and H that gives the correct Z₀
- For S-parameter analysis of CPW devices: load measured .s2p files directly — format is the same as microstrip
CPW Measurement Considerations
- On-wafer CPW structures measured with GSG probes at wafer level
- VNA calibration: TRL on the same CPW substrate for accurate reference plane
- Mode conversion: CPW can excite substrate modes — use GCPW (with bottom ground) to suppress
RF View CPW Analysis: Load CPW structure .s2p files for full S-parameter analysis — same Touchstone format as microstrip. All RF View analysis features (Smith chart, matching, simulation) work identically for CPW S-parameters. Free on Android.