Grounded Coplanar Waveguide
Understanding Grounded Coplanar Waveguides (CPWG)
While standard Coplanar Waveguides (CPW) place all conductors on the top surface of the dielectric, leaving the bottom bare, this design is mechanically fragile and offers no heat sinking for active components. By adding a solid ground plane to the bottom of the substrate, engineers create the Grounded Coplanar Waveguide (CPWG).
Electromagnetic Behavior and Impedance
In a standard CPW, the electric fields arch entirely between the center trace and the top ground planes. When a bottom ground plane is introduced, some of the electric field lines are pulled downward through the dielectric substrate. The CPWG operates in a quasi-TEM mode that is a complex hybrid between a standard CPW and a microstrip.
The characteristic impedance ($Z_0$) of a CPWG is determined by three interacting variables:
- Trace Width ($W$): The width of the central signal line.
- Gap Width ($G$): The distance to the top ground planes.
- Substrate Height ($h$): The distance to the bottom ground plane.
If the gap $G$ is very small compared to the height $h$, the line behaves mostly like a pure CPW. If the gap $G$ is very large, the top ground planes have little effect, and the line behaves exactly like a microstrip.
The Critical Role of Via Stitching
The biggest danger of using CPWG is the creation of parallel-plate waveguide modes. Because there are two massive ground planes separated by a dielectric (the top grounds and the bottom ground), high-frequency RF energy can couple into this structure, resonating and leaking power away from the signal trace.
To suppress this, engineers must tie the top and bottom ground planes together using dense rows of plated through-holes (vias), known as via stitching or a "via fence."
| Via Stitching Rule | Electromagnetic Reason |
|---|---|
| Proximity to Trace | The vias must be placed as close to the gap edge as manufacturing tolerances allow to confine the fields and prevent energy from spreading outward. |
| Spacing Between Vias | The pitch (distance) between adjacent vias must be strictly less than $\lambda/4$ (and ideally $< \lambda/10$) at the highest operating frequency to prevent leakage through the "fence." |
| Symmetry | The via placement must be perfectly symmetrical on both sides of the trace to prevent the excitation of unwanted coupled slotline modes. |
Key Equations
A Grounded Coplanar Waveguide (CPWG), also known as Conductor-Backed Coplanar Waveguide (CBCPW), is a high-frequency planar transmission line that features a signal trace and two...
Key specifications:
0 dB | 1 mW | 30 dB | 1 W | 110 GHz | 50 dB
Z0: = √(L/C) = √((R+jωL)/(G+jωC))
Comparison
| Aspect | Grounded Coplanar Waveguide Spec | Typical Range | Impact | Design Note |
|---|---|---|---|---|
| Primary function | This architecture provides excellent ele... | Application-dep. | Critical | Verify in sim |
| Operating range | By adding a solid ground plane to the bo... | Application-dep. | Critical | Verify in sim |
| Performance | Electromagnetic Behavior and Impedance I... | Application-dep. | Critical | Verify in sim |
| Integration | When a bottom ground plane is introduced... | Application-dep. | Critical | Verify in sim |
| Trade-off | The CPWG operates in a quasi-TEM mode th... | Application-dep. | Critical | Verify in sim |
Frequently Asked Questions
Why use CPWG instead of Microstrip for power amplifiers?
High-power RF transistors (like GaN HEMTs) generate massive amounts of heat. In a microstrip, the heat must travel through the poorly-conductive dielectric substrate to reach the heatsink. In a CPWG, the transistor can be mounted directly to the top ground planes, which are thermally tied to the bottom ground plane via massive copper arrays, providing vastly superior thermal dissipation.
Is CPWG more lossy than standard CPW?
Generally, yes. Because more of the electric field is forced to travel through the dielectric substrate (to reach the bottom ground plane) rather than through the air above, CPWG suffers from slightly higher dielectric attenuation ($\alpha_d$) than un-grounded CPW.
How does CPWG help with dense circuit routing?
Because the electromagnetic fields are tightly confined by both the top and bottom ground planes, CPWG offers exceptional isolation (low cross-talk). This allows engineers to route multiple high-frequency RF traces much closer together on the PCB without them interfering with each other.