Signal Integrity / A Case Study

Introduction

Arira Design’s Signal Integrity Group was requested to re‐design an existing 5GHz Grounded Coplanar Waveguide (GCPW) RF feedline to improve the performance of a WiFi subsystem on the client’s board. Measurements showed that the impedance of the feedline impedance was approximately 38 Ohms.

Ansys

 

 

 

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

1 Failure to account for the effects of soldermask on the trace impedance

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

3 Incorrect cutout in a nearby non‐reference ground plane

The existing feedline was simulated and the coplanar geometry was improved based on simulation results to meet the impedance requirement of 50 Ohms. The client reported greatly improved WiFi performance with the new PCB.

This paper discusses the coplanar geometry of the initial PCB design, the effects of the three items shown 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 and Grounded Coplanar Waveguides.

Grounded Coplanar Waveguides

Grounded Coplanar Waveguides are becoming more prevalent on PCB designs due to the pervasiveness of WiFi and BT integration on modern PCBs. Some of the advantages of GCPW over traditional microstrip transmission lines are:

  • 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 WiFi 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 vias.

Many tools are available to calculate the impedance of GCPW structures, but the free tools that are available on the Web 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

PCB Description

The PCB under consideration is a high‐volume four‐layer board for a consumer product fabricated using an FR‐4 dielectric with a nominal Dk of 4.2 . The board thickness was approximately 45 mils. The GCPW was on layer 1 with the ground reference island on layer 3. Layer 2 was a ground plane layer. Layer 3 was a power plane layer that also had the ground reference island for the GCPW, and layer 4 was a signal layer. Through‐vias with a diameter of 8 mils connected the layer 1 ground planes with the layer 3 reference plane and the main ground plane on layer 2.

The Grounded Coplanar Waveguide geometry on the initial PCB was designed by using a freeware transmission line calculation tool found on the Web. The trace width of the GCPW was approximately 24 mils. That trace width was chosen to match the pin size of the WiFi module and the antenna connector. The trace thickness after plating was approximately 1.5 mils. The coplanar trace was located approximately 37 mils above the ground reference plane.

With this input, the transmission line calculation tool initially used by our client determined the coplanar gap to be 3 mils. There was a cutout on the layer 2 ground plane. The PCB designer matched the width of this cutout to the width of the coplanar ground relief on layer 1 so that the cutout in the layer 2 plane was 30 mils in width. Please see the figure below.

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