When to Use Coplanar Waveguide Routing for HF Boards

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When to use coplanar waveguide routing and layout

Radar systems, wireless systems, high frequency analog systems…all of these need to include measures to ensure signal integrity. With many high frequency systems, this can be difficult with microstrip or embedded microstrip routing on the surface layer. However, you can save yourself a lot of signal integrity headaches using coplanar waveguide routing.

Most design tools can be used to define coplanar waveguide routing with a ground pour feature. This allows you to easily define a coplanar waveguide on your board, or a grounded coplanar waveguide by defining a ground plane in an interior layer. The question remains: when should you use coplanar waveguide routing? Here’s everything you need to know.

What is Coplanar Waveguide Routing?

A coplanar waveguide is a copper arrangement where a signal trace is routed in parallel to two ground planes. The presence of the ground plane on each side of a signal trace provides natural shielding for the signal against interference from other traces on a board. A coplanar waveguide also comes in the grounded variety. The geometry is essentially the same, except there is another ground plane beneath the surface layer. This is shown in the image below.

Different signal and ground plane arrangements in coplanar waveguide routingCoplanar waveguide geometries

Advantages of Coplanar Waveguide Routing

Compared to microstrip and stripline traces, placing a signal trace on the top layer with the ground pour on each side of the trace causes a signal to see lower radiation losses. This also reduces resistive heating losses as the signal hugs the side of the trace, rather than hugging the bottom of the trace near the rough interface with the substrate. This means your signal will be stronger at the receiver end of the trace, and the shape of your signals will not be distorted as they travel along the trace.

Most feedlines, for example Bluetooth and WiFi transceivers and antennas, require series and/or shunt elements for impedance matching. Because coplanar waveguides have a ground plane directly next to the trace, these parallel components can be mounted directly between the trace and the ground plane without placing routing through a via.

Disadvantages of Coplanar Waveguide Routing

Because coplanar waveguides require the use of ground planes surrounding the trace, you have less real estate available on the surface layer. The cost of all that copper on the surface layer also drives up the board cost. You also need a relatively thick substrate, so you should keep the layer count low if you are using a standard board thickness.

There are closed-form equations for the impedance of a coplanar waveguide, but these formulas require evaluating elliptical integrals. If you’re more focused on the design than the math, you probably don’t have time to go and calculate the solutions to these equations. One thing you will notice is that the impedance is more sensitive to the spacing between the signal trace and the ground plane than it is to the cross-sectional geometry of the signal trace.

This means you’ll need to use a calculator that calculates the impedance numerically. Thankfully, you can find a number of calculators online for specific coplanar waveguide geometries, including a grounded coplanar waveguide. There is a great calculator on Sourcefourge that allows you to consider everything from your substrate dielectric properties and the frequency you will work with in your board.

There is another issue with coplanar waveguides that relates to the plating used on copper to prevent trace corrosion. Electroless nickel immersion gold (ENIG) plating has higher insertion loss on a coplanar waveguide than on a microstrip. The alternative coating, hot air solder leveling, has lower insertion loss but will have a rougher surface, leading to greater losses in traces. Unless you are working near 100 GHz, either surface finish will likely be just fine for your application, as long as your trace lengths are not too long.

Large PCB with ENIG surface finish
ENIG surface finish on a PCB

When to Use Coplanar Waveguide Design

Many designers have jumped head-first into Bluetooth-capable devices, but working with coplanar waveguide routing gives you an easy way to get into working with higher frequency devices (i.e., 5 GHz and above) while ensuring signal integrity throughout the board. Some example applications include radar systems, both for automotive and UAV projects, and even 5G-capable devices. The price point for components for these devices has been dropping recently, and now anyone can get into the game with coplanar waveguide routing.

There is no specific frequency limit at which you should switch from microstrip to coplanar waveguide layout and routing. However, industry types are commonly using coplanar waveguide routing in applications that operate at 10s of GHz, and in systems that require ultra-precise signal integrity (think a guidance system for a missile). If you’re doing anything above 5 GHz, then you can consider using coplanar waveguides on your most sensitive analog signals in order to isolate them from nearby digital signals. If you are working on a fully analog board at these same frequencies, then you should probably use coplanar waveguide routing.

High Frequency Design in Upverter

Working with Upverter’s browser-based PCB design platform allows you to easily implement a coplanar waveguide routing strategy thanks to the ground pour feature in the PCB layout tools. If you’re designing a grounded coplanar waveguide system, you’ll also have the via design tools you need to provide the connection between surface and interior ground planes in your board.

Alternatively, if you want to work with an existing Arduino project for a high frequency board, you can find some great open-source 24 GHz radar projects for high frequency radar that will work with an Arduino. This particular project is based on the XMC1302 and XMC4200 ICs from Infineon, and the author included their ARM code for programming the Arduino controller. This type of project is ideal for a UAV that will incorporate chirped radar. 

24 GHz radar board for Arduino with coplanar waveguide layout and routing
An
Arduino 24 GHz radar project

With the browser-based design features in Upverter®, you’ll have access to the PCB design features you need to create high speed or systems or high frequency projects that use coplanar waveguide layout and routing. The schematic design and PCB layout tools are designed for taking your design from start to finish and preparing for manufacturing. These standard design features are accessible from anywhere. These features also provide collaboration and rules verification for your next embedded systems project.

You can sign up for free and get access to the best browser-based PCB editor, schematic editor, and component database. Visit Upverter today to learn more.

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