Day 3 - Theory of High-Speed Design [e-blog] : The Common Types of Transmission Lines

 

Day 3: The Common Types of Transmission Lines

Now that you understand a trace is a transmission line and that its impedance matters, let's look at the most common types of transmission lines you'll encounter on a PCB. The geometry of the trace determines its impedance and performance.

1. Microstrip:

   • Description: A trace that is on an outer layer of the PCB, with a solid ground or power plane directly beneath it on an inner layer.

   • Pros: Easy to route and inspect.

   • Cons: The electromagnetic field extends into the air above the trace, which can lead to more EMI (Electromagnetic Interference) and is less stable in terms of impedance control.

2. Stripline:

   • Description: A trace that is on an inner layer of the PCB, sandwiched between two solid reference planes (usually a ground plane above and below).

   • Pros: The electromagnetic field is fully contained between the planes, leading to better EMI performance and very stable, predictable impedance.

   • Cons: Requires more layers and is more difficult to route and inspect.

3. Co-planar Waveguide:

   • Description: A trace that has a ground plane on the same layer, next to it, and often has a solid ground plane beneath it as well.

   • Pros: Provides excellent signal isolation and impedance control, especially for RF and very high-speed designs.

   • Cons: Requires more routing space due to the need for adjacent ground pours.

For your designs, you will be using a combination of these. Your high-speed signals (like DDR or PCIe) will likely be routed as striplines for the best performance, while less critical signals might be routed as microstrips.

---

Book Reference for Deeper Study

Both books are considered classics in the field, but they serve different purposes. Think of it this way:

• PCB Currents: How They Flow, How They React by Douglas Brooks is the masterclass in the physics of current flow on a PCB. It's fantastic for understanding the "why"—why do return paths matter, why does current follow the path of least impedance at high frequencies, and how do physical properties affect signal behavior? This book will build your fundamental understanding of the core concepts behind every high-speed problem.

• High-Speed Digital Design: A Handbook of Black Magic by Howard Johnson and Martin Graham is more of a practical "how-to" handbook. It focuses on solving real-world design problems, such as how to terminate traces, how to design a proper power delivery network, and how to deal with specific high-speed interfaces like DDR. It provides more of a direct, step-by-step guide to the design process itself

Share on Google Plus

About Praveen S

Hello everyone, I am Praveen S, currently employed as "Technical Design Engineer" for Polycab Wires Pvt Ltd , I am a BE Graduate in EEE from NMAMIT Nitte (Karnataka, India). I am very passionate about learning and sharing my knowledge relating to the areas of "Electrical and Electronics". Here's my blog to share my ideas and to improve myself.