Optimizing the RF feedline in PCB design

Editor's Note: Wireless design can stymie the best laid plans for connected device development. In particular, an improperly designed antenna feedline can be difficult to discover until late in development during testing. Here's a taste of a nice article from our friends at EEWeb, offering an in-depth look at an approach used to improve the design of a grounded coplanar waveguide RF feedline needed to improve Wi-Fi performance. 

Recently, the Signal Integrity Group at Arira Design was requested to re‐design an existing 5GHz Grounded Coplanar Waveguide RF feedline to improve the performance of a Wi-Fi subsystem on the client's board. Measurements showed that the impedance of the feedline impedance was approximately 38 ohms.

Prior to simulation, several issues were uncovered with the original design including:

  • Failure to account for the effects of the solder mask on the trace impedance

  • Failure to account for PCB etchback in the trace impedance calculation

  • Incorrect cutout in a nearby non‐reference ground plane

The existing feedline was simulated, after which the coplanar geometry was improved based on the simulation results to meet the impedance requirement of 50 ohms. As a result, the client reported greatly improved Wi-Fi performance with the new PCB.

This paper discusses the coplanar geometry of the initial PCB design, the effects of the three items noted above, and the final coplanar geometry. E‐Field plots are shown for different coplanar configurations to illustrate the intentional and unintentional coupling that can occur with grounded coplanar designs (it is assumed that the reader is familiar with the basic structure of Coplanar Waveguides, or CPWs, and Grounded Coplanar Waveguides, or GCPWs).

Grounded Coplanar Waveguides

Grounded Coplanar Waveguides are becoming more prevalent in PCB designs due to the pervasiveness of Wi-Fi and Bluetooth integration on modern circuit boards. Some of the advantages of GCPW over traditional microstrip transmission lines are as follows:

  • Lower loss: More E‐field lines travel through the air as opposed to flowing through the lossy PCB material. This can enable the use of less expensive FR‐4 for PCB designs operating at 5GHz.

  • Isolation: GCPW lines offer more isolation compared to microstrip because the field lines are more tightly confined.

  • Flexible Geometry: The GCPW impedance is primarily controlled by the gap between the trace and the coplanar ground structure. This enables more flexibility in trace widths compared to microstrip transmission lines.

  • Lower Copper Surface Roughness Loss: The current in microstrip lines tends to concentrate along the bottom of the trace, which is where the copper is roughest (to promote adhesion to the dielectric). Properly designed GCPW transmission lines tend to have the current concentrated on the edges of the trace, where the surface is smoother.

  • Superior Matching Component Placement: Most Bluetooth or Wi-Fi RF feedlines require series and/or parallel matching components. Because the GCPW has the ground immediately adjacent to the trace, the parallel components can be mounted directly between the trace and the coplanar ground, which eliminates the parasitics associate with the vias.

Many tools are available to calculate the impedance of GCPW structures, but the free tools that are available on the Internet typically have restrictions on the types of structures that can be analyzed. Basic structures can usually be calculated, but the effects of nearly copper structures usually need EM simulation to model them correctly.

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