Design RF PCBs With a Coplanar Waveguide Calculator

Zachariah Peterson
|  Created: February 1, 2021
Design RF PCBs With a Coplanar Waveguide Calculator

RF PCBs can be complex boards that need careful interconnect design and layout. If you’re designing an RF PCB for microwave or higher frequencies, controlled impedance routing should be top of mind to ensure signal integrity. You can choose from many routing styles, but whichever routing style you choose, it should provide isolation against noise and crosstalk.

A coplanar waveguide routing style is an excellent choice for RF PCBs as they can be designed to have low dispersion, high isolation, and less loss than striplines. To properly design coplanar waveguides, you’ll need a specialized coplanar waveguide calculator. Instead of using textbooks or complex field solvers, try using the complete set of interconnect design features in Altium Designer. You’ll have access to a simple field solver built into your PCB layer stack design tools and a complete set of routing tools for your layout.

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RF layout and routing is an art form that is starting to become more critical for digital designers. Laying out a board with digital and RF sections requires ensuring isolation between different circuit blocks with smart floorplanning. In addition, traces need to be carefully routed while ensuring return paths do not cross to create interference. The constraints in RF boards can be challenging, even for the experienced digital designer. Coplanar waveguide routing is one option that satisfies these constraints and provides other benefits in RF PCBs.

When it comes to coplanar waveguide design and routing, designers need to ensure strict impedance control, especially at microwave or mmWave frequencies. When your PCB design software includes a coplanar waveguide calculator, you can determine the proper width and spacing in a coplanar configuration as part of impedance-controlled routing. This is exactly what you’ll find when you use the RF interconnect design and routing tools in Altium Designer.

What is a Coplanar Waveguide Calculator?

A coplanar waveguide is a simple routing style where a trace is routed on the surface layer and surrounded by ground pour on each side. In addition, routing is done above a ground plane, providing high isolation in the PCB layout. In addition, a via fence can be used to tie the ground pour back to the ground plane for even higher isolation, known as grounded coplanar waveguide routing. Both varieties are shown below.

Designing a coplanar waveguide

Coplanar waveguide design

When you need to implement impedance control in your PCB, you’ll need to determine the PCB trace width that best suits your design and ensures your trace impedance takes the correct value. This is where a coplanar waveguide calculator becomes important. The designer can normally enter their desired impedance, and the calculator will determine the PCB trace width needed to meet that impedance goal.

Factors Affecting Coplanar Waveguide Impedance

When calculating coplanar waveguide impedance, there are many factors that affect the waveguide’s impedance, losses, and isolation:

  • Geometry: The width of the central trace and the spacing between the trace and ground pour will determine the field distribution around the trace, which then determines how much loss signals experience and the level of isolation provided by the surface ground pour.
  • Substrate dielectric constant: The dielectric constant of the substrate has dispersion, which will determine the amount of loss seen at different frequencies. Dispersion also determines how different frequencies travel at different speeds.
  • Copper roughness: At high frequencies (microwave and mmWave), copper roughness will increase the impedance of an interconnect. Most online calculators do not account for copper roughness.
  • Skin effect: The skin effect in a PCB trace, including coplanar waveguides, adds additional AC and DC resistance to the total impedance.

The best coplanar waveguide calculator can account for all of these effects simultaneously and without a complex 3D field solver. The best PCB stackup design and routing tools will include these calculations to help designers ensure impedance control throughout the required bandwidth for their signals.

Coplanar waveguide routing in Altium Designer

Designing coplanar waveguide geometries is easy with the powerful routing tools in Altium Designer.

Using a Coplanar Waveguide Calculator

A coplanar waveguide calculator will operate in one of two ways. Either the desired impedance at a specific frequency is used to determine the waveguide width, or the width is entered and the impedance is calculated. In both cases, you’ll need to enter your stackup information into the calculator to get accurate results. These calculators will also return the effective dielectric constant seen by signals propagating on the waveguide.

The problem with online coplanar waveguide calculators is that they do not include dispersion, copper roughness, skin effect, or loss tangent in the PCB substrate. The major problem with neglecting dispersion is that the impedance calculated at one frequency may no longer be correct at any other frequency. Therefore, these calculators are only useful for a single frequency and cannot treat broadband signals in real PCBs. This is why designers need an advanced coplanar waveguide calculator for high-frequency PCBs.

Advanced Coplanar Waveguide Calculator Functions

More advanced coplanar waveguide calculator utilities will include all the effects mentioned above when calculating the impedance and effective dielectric constant of the waveguide. In addition, they can consider the dielectric constant of the PCB substrate throughout your signal’s bandwidth. This gives you an ultra-accurate impedance and width calculation, which can then be used in your PCB layout and routing tools.

PCB stackup design and coplanar waveguide calculator in Altium Designer

Sizing traces for controlled impedance with Altium Designer’s coplanar waveguide calculator.

Altium Designer Has the Best Impedance Calculator

Forget about using online calculators to size your waveguide and determine impedance. The impedance calculator in Altium Designer integrates with your PCB layer stack design tool and can calculate impedance for many transmission line geometries, including coplanar waveguides. Once you’ve calculated your waveguide impedance, the interactive routing features in Altium Designer can automatically apply your waveguide sizing rules while routing your PCB.

Complete RF PCB Design and Layout in Altium Designer

In addition to a powerful coplanar waveguide calculator, Altium Designer users will have access to a complete set of PCB layout and routing tools for advanced electronics. These routing features take information directly from your impedance calculation and layer stackup, helping you stay productive as you create your PCB. Once your new RF PCB layout is complete, you can prepare PCB fabrication deliverables for your manufacturer. Only Altium Designer contains a complete set of tools for PCB design, layout, and manufacturing.

PCB design rule violation in Altium Designer

Quickly spot design errors in your PCB layout when you use the complete set of rules-driven design tools in Altium Designer.

Forget about running interconnect calculations by hand or using weak online calculators. Use the best interconnect design and analysis tools in Altium Designer, the only platform that unifies design, layout, manufacturing, and analysis.

Altium Designer on Altium 365 delivers unprecedented integration to the electronics industry until now relegated to the world of software development, allowing designers to work from home and reach unprecedented levels of efficiency.

We have only scratched the surface of what is possible to do with Altium Designer on Altium 365. You can check the product page for a more in-depth feature description or one of the On-Demand Webinars.

About Author

About Author

Zachariah Peterson has an extensive technical background in academia and industry. He currently provides research, design, and marketing services to companies in the electronics industry. Prior to working in the PCB industry, he taught at Portland State University and conducted research on random laser theory, materials, and stability. His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental sensors, and stochastics. His work has been published in over a dozen peer-reviewed journals and conference proceedings, and he has written 1000+ technical blogs on PCB design for a number of companies. He is a member of IEEE Photonics Society, IEEE Electronics Packaging Society, and the American Physical Society, and he currently serves on the INCITS Quantum Computing Technical Advisory Committee.

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