Optimizing RF Feeders in PCB Design

Infineon / Mitsubishi / Fuji / Semikron / Eupec / IXYS

Optimizing RF Feeders in PCB Design

Posted Date: 2024-01-18

Wireless designs may hinder plans for connected device development. In particular, poorly designed antenna feeds are difficult to detect during testing late in development. Here's a nice article from our friends at EEWeb that provides an in-depth look at a method for improving grounded coplanar waveguide RF feeder design to improve Wi-Fi performance.

Arira Design's signal integrity team was asked to redesign an existing 5GHz grounded coplanar waveguide RF feedline to improve the performance of the Wi-Fi subsystem on the client's board. Measurements show that the impedance of the feeder impedance is approximately 38 ohms.

Prior to simulation, several issues were discovered in the original design, including:

Failure to consider the impact of solder mask on trace impedance

Failure to account for PCB etchback in trace impedance calculations

Incorrect cutouts in nearby non-reference ground planes

The existing feeder was simulated and the coplanar geometry was improved based on the simulation results to meet the 50 ohm impedance requirement. As a result, customers say the new PCB greatly improves Wi-Fi performance.

This article discusses the coplanar geometry of the initial PCB design, the effects of the above three items, and the final coplanar geometry. Electric field plots for different coplanar configurations are shown 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 waveguide

Due to the popularity of Wi-Fi and Bluetooth integration on modern circuit boards, grounded coplanar waveguides are becoming increasingly common in PCB designs. Some advantages of GCPW over traditional microstrip transmission lines are as follows:

Lower losses: More electric field lines travel through the air than through lossy PCB materials. This allows PCB designs operating at 5GHz to use cheaper FR-4.

Isolation: GCPW lines provide more isolation than microstrip lines because the field lines are more tightly restricted.

Flexible Geometry: GCPW impedance is primarily controlled by the gap between traces and coplanar ground structures. This allows for more flexibility in trace width compared to microstrip transmission lines.

Lower copper surface roughness losses: Current in microstrip lines tends to concentrate along the bottom of the trace, where the copper is rough (to promote adhesion to the dielectric). Properly designed GCPW transmission lines tend to concentrate current at the edges of the trace, where the surface is smoother.

Superior Matching Component Placement: Most Bluetooth or Wi-Fi RF feeders require series and/or parallel matching components. Because the GCPW's ground is in close proximity to the trace, parallel components can be mounted directly between the trace and coplanar ground, eliminating via-related parasitics.

Many tools are available for calculating the impedance of GCPW structures, but the tools available on the Internet often have limitations on the types of structures that can be analyzed. The basic structure can usually be calculated, but the effects of near-copper structures often require EM simulations to model correctly.

#Optimizing #Feeders #PCB #Design