High-Speed Routing

"High-speed routing" is the field of PCB (Printed Circuit Board) design that is in charge of arranging copper traces on a PCB in a way that minimizes unwanted parasitic effects that can occur with fast signals. For example, if you are routing an HDMI signal capable of transmitting 3Gbps of data, the standard recommends single-ended impedance of 50 Ohms, differential impedance of 100 Ohms, differential pairs with matching lengths within 3 mm, etc. This is difficult to achieve in most PCB designs; so, traces should be properly designed for high-speed routing to avoid issues.

Many consumer electronics revolve around chips, microcontrollers, digital audio processing, etc. However, select applications use high-frequency signals or even high-frequency clock references. It is in this scenario that high-speed routing problems may occur. In a printed circuit, all conductors act as antennae, which means that they can transmit or receive waves to/from other traces on the board (or even other circuits).

The most common aspects to keep in mind when designing a high-speed PCB are:

• Impedance: When dealing with high-frequency signals, traces on the board stop acting as simple conductors and behave as transmission lines. As such, they exhibit a characteristic impedance that will determine the way a wave propagates through them (how much of it is transmitted, reflected, etc.).
• Parasitic components: In a PCB full of conducting traces, planes, and dielectrics, the disposition of these materials in the presence of moving charges can simulate the effects of passive elements. Resistance is the most straight-forward one since it’s related to impedance, which was discussed before. However, stray capacitors and inductors also appear. The former might be seen when two conducting surfaces face each other. The latter are seen in vias and the traces themselves, for example.
• Crosstalk: Being one of the most common issues, seeing interference of one signal in another one’s path is provoked by how close two traces are from each other. The connection occurs through a parasitic capacitor between the two lines. Depending on the intensity of the signals, it can also be caused by a trace radiating energy onto others.
• Inherent transmission line effects: Having signals travel a transmission media with non-ideal conditions implies that the resistance and inductance of the traces will attenuate the wave, the unmatched impedance of the connections (trace to trace, trace to connector, etc.) will produce reflections, and the unmatched length of different traces will cause a relative delay between signals that might significantly affect the effectiveness of the information, amongst others.

However, there are several ways to prevent or mitigate all these problems. There are some guidelines to follow when routing a PCB that has high-speed signals:

• The further apart two traces are, the less crosstalk there will be between them. Separating traces by a distance equal to 3 times their width is a common rule of thumb.
• Keep amplified signals away from each source.
• Try to have uniform ground planes under the traces, ensuring a continuous return path, and avoid having high-speed traces run under other ones.
• Use bypass capacitors and keep their traces as short as possible to mitigate the effects of the parasitic inductor.
• Avoid 90-degree angles in your routing.
• Match the length of signals that need to be synchronized.

PCB Editor offers a variety of tools that help designers in their high-speed routing. Opening the Workflow Manager (Analyze > Workflow Manager) will display a set of procedures that you can use when dealing with these situations.

For more information about these functions, please visit our learning website.

Team PCBTech

Cadence Design Systems