Harness mmWave Routing with a Mode-Selective Transmission Line

Zachariah Peterson

|  October 22, 2020

High speed PCBs are pushing data rates into the stratosphere, putting tight design requirements on interconnects to ensure signal integrity and low losses. In an earlier article, I discussed substrate integrated waveguide routing for RF PCBs in an earlier article as one option for high frequency routing. This type of transmission line provides excellent isolation and is useful for simple transitions to antennas, but it’s not the only option for routing high frequency designs.

A mode-selective transmission line is one variation on a coplanar waveguide configuration for routing signals between components at very high frequencies. The goal in using a mode-selective transmission line or other geometry is to provide low dispersion and low loss routing in specific bandwidths with single-mode propagation. In this article, I’ll present this simple variation on coplanar waveguides and how you can use mode-selective transmission lines to provide high isolation routing with mode selection for RF applications.

High Speeds Bridge the Gap with RF Design

Whether you’re a digital designer or an RF designer, the push of high speed digital channels to higher frequencies is forcing everyone to take on RF concepts during design. John Coonrod, who happens to be one of my favorite speakers on this important topic, states very eloquently that concepts from RF design will be critical for digital signal integrity as we get ever closer to 1 ps rise times in practical applications and the limits of TEM transmission lines. In particular, as signal frequencies increase, the transverse modes in conventional microstrip or stripline traces will be excited, which is undesirable for both digital and RF routing.

For these reasons, some waveguide geometries may be more ideal at very high frequencies and for very high data rate applications as they can be designed to allow single-mode routing. Some of these alternative routing geometries are:

• Substrate integrated waveguide. Take a look at this article to learn more about these structures. I’ve used these with a slot in the surface to couple to a conventional microwave waveguide flange. 
• Rectangular stripline waveguide. This waveguide style has a stripline routed inside a substrate integrated waveguide. The via wall in this waveguide provides high isolation and allows diverse mode selection. 
• Mode-selective transmission line. This is a variation on coplanar waveguide routing and provides many of the same advantages.

If you look in the research literature, these alternative routing styles have been around for a long time and have shown their feasibility for routing up to hundreds of GHz. These waveguide structures are simple to produce with standard fabrication techniques, but even they have limits once we get to extremely high frequencies. Among these, a mode-selective transmission line (MSTL) can be easily produced with a grounded coplanar waveguide geometry (GCPW) as shown below.

This simple change in the width between vias isn’t the only difference between a grounded coplanar and mode-selective transmission line, but it does transition the structure from quasi-TEM to TE propagation. Each higher order mode in the structure has a cutoff frequency, and simply exciting the structure above a cutoff will cause the electromagnetic field to propagate through the structure at the desired mode.

In a typical microstrip or stripline, you will eventually excite the parallel plate waveguide modes. Unfortunately, in these geometries, there is no way to suppress these modes except to make the laminate thinner, which will eventually reach its limit and isn’t applicable in all designs. This possibility of higher order mode excitation is one of the fundamental limits on TEM transmission lines.

In contrast, waveguides can be tuned to allow or suppress various modes by selecting the appropriate geometry. The structure of a mode-selective transmission line gives it the following characteristics:

• High isolation. This is the primary benefit of routing in any waveguide, including a mode-selective transmission line. The grounded via fence along the edge provides shielding from other traces.
• Mode suppression. Modes in a mode-selective transmission line are a combination of a substrate integrated waveguide mode and quasi-TEM mode for the center conductor. The via fence spacing along the edge can be used to suppress substrate waveguide modes to ensure single-mode propagation.
• Broadband low dispersion. By suppressing a surface wave, you’ve ensured flat dispersion out to a higher bandwidth than in a microstrip or stripline. By keeping dispersion low over a broader bandwidth, there is less distortion and deviation from target impedance, which also suppresses intersymbol interference seen at a receiver. 

Routing a Mode-selective Transmission Line in Your PCB

Routing a coplanar waveguide geometry like a mode-selective transmission line takes the right set of CAD tools. Here’s a simple procedure to route these lines:

1. Calculate the wave impedance for the desired mode you’ll be working with. To do this, simply calculate the required propagation constant and use the standard wave impedance equation from an RF design textbook.
2. Set the required clearance between your mode-selective transmission line nets and any nearby grounded polygons.
3. Fill in the area around your routed nets with grounded copper.
4. Place stitching vias on the selected polygon.

This transmission line geometry has been shown to allow terabit per second data transmission and it may soon become a critical part of the high speed design landscape. To learn more about the theory of mode-selective transmission lines, read this paper from IEEE.

If you want to design ultra-high speed/high frequency designs with advanced routing geometries, you need the PCB design and layout tools in Altium Designer®. You’ll have the routing and layout features you need to route mode-selective transmission lines and other waveguide geometries with ease.

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