Introduction to High Speed PCB Designing: Techniques for Avoiding Crosstalk

Created: November 8, 2017
Updated: December 28, 2020
Introduction to High Speed PCB Designing: How to Eliminate Crosstalk

Recently at a wedding reception, I was trying to talk to a gentleman who was sitting at the same table as me. Unfortunately, there was a woman sitting between us carrying on a conversation with someone else sitting on my other side. With all the commotion of the reception in the background, the conversation was difficult, to begin with. Having another discussion taking place between us though made our conversation impossible. What we had was crosstalk!

Crosstalk during a conversation can be very annoying, but crosstalk in PCB layouts can be disastrous. If not corrected, crosstalk can cause your finished circuit board to either not work at all, or maybe be plagued by intermittent problems. Let’s take a look at what crosstalk is and how to prevent crosstalk in your high-speed PCB design in this introduction to high-speed PCB designing.

What is Crosstalk in our Introduction to High-Speed PCB Designing?

Crosstalk in PCBs is the unintentional electromagnetic coupling between traces. This coupling can cause the signal pulses of one trace to overpower the signal of the other trace even though they are not physically touching each other. This can happen when the spacing between parallel traces in PCB is tight. Even though the traces may be maintaining the minimum spacing for manufacturing purposes, they may not be enough for electromagnetic purposes.

Consider two traces in PCB running parallel to each other. If the signal in one trace has more amplitude than the other, it could aggressively influence the other trace. The signal in the “victim” trace will then begin to mimic the characteristic impedance of the aggressor trace instead of conducting its own signal. When this happens, you have crosstalk.

Crosstalk in PCBs is usually thought of as happening between two parallel traces running next to each other on the same layer. There is an even greater possibility, though, for crosstalk in PCB to occur between two parallel traces running next to each other on adjacent layers. This is called broadside coupling, and it is more likely to happen because the two adjacent signal layers are separated by a very small amount of core thickness. This thickness can be 4 mils (0.1 millimeters) which is sometimes less than the spacing between two traces on the same layer.

Blue highlighted traces on a black and white circuit board
Trace spacing to eliminate crosstalk is typically larger than regular trace spacing requirements

Removing the Potential for Crosstalk from Your High-Speed Design

Fortunately, you are not at the mercy of crosstalk in PCB. By having a high-speed PCB design that is focused on minimizing the potential of crosstalk situations, you can avoid these problems. Here are some high-speed design techniques for avoiding crosstalk that will help you to eliminate any chance of it appearing on your board:

  1. Keep as large of a distance as possible between differential pairs and another signal routing. The rule of thumb is gap = 3 times the trace width.

  2. Keep as large of a difference as possible between clock routing and another signal routing. The same gap = 3 times the trace width rule of thumb works here as well.

  3. Keep as large of a distance as possible between different sets of differential pairs. The rule of thumb here is slightly larger, gap = 5 times the trace width.

  4. Asynchronous signals (like RESET, INTERRUPT, etc.) should be routed away from buses and high-speed signals. They can be routed next to on-and-off or power-up signals though because those signals are rarely used during the normal operation of the circuit board.

  5. Make sure that two signal layers adjacent to each other in the board stack-up will alternate horizontal and vertical routing directions. This will reduce the chance of broadside coupling by not allowing the traces to run parallel on top of each other.

  6. A better way to reduce potential crosstalk between two adjacent signal layers is to separate the layers with a ground plane layer between them in a microstrip configuration. Not only will the ground plane increase the distance between the two signal layers, but it will also provide the required return path for the signal layers.

Picture of computer screen with hand holding a wrench coming out of it
Your design tools and third-party applications can help you to eliminate crosstalk in PCB designs

Introduction to High-Speed PCB Designing: Design Software that Helps You Eliminate Crosstalk

PCB design tools have a lot of functionality built into them to help you to avoid crosstalk in your designs. Circuit board layer rules will help you to avoid broadside coupling by specifying routing directions and creating microstrip stack-ups. With net class rules, you will be able to assign greater trace spacing to groups of nets that are more susceptible to crosstalk in PCB. Diff pair routers will route your differential pairs together as an actual pair instead of routing them individually. This will maintain the required spacing of the differential pair traces to each other and to other nets in order to avoid crosstalk.

In addition to the built-in functionality of your high-speed PCB design software, there are also other tools that can help you to eliminate crosstalk in high-speed PCB design as well. There are different crosstalk calculators available to help you determine the proper trace width and spacing for your routing. There are also signal integrity simulators to analyze your high-speed design for potential crosstalk problems.

Crosstalk can be a big problem on a printed circuit board if allowed to happen. Now that you know what to look for though, you will be prepared to prevent crosstalk in PCB from happening. The design tips that we have discussed here along with the functionality in your high-speed PCB design software will help you to create a crosstalk-free design.

PCB design software, like Altium Designer, has the advanced functionality that we’ve discussed here already built into it. Would you like to find out more about how Altium Designer can help you to work through crosstalk and other signal integrity issues in your PCB design? Want more info for your introduction to high-speed PCB designing? Talk to an expert at Altium Designer.

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