How to reduce crosstalk in PCB design?
Crosstalk is unintentional electromagnetic coupling between traces on printed circuit board. This coupling can cause the signal pulses of one trace to exceed the signal integrity of another trace, even if they are not in physical contact. This happens when the spacing between parallel traces is tight. Even though the traces may be kept at a minimum spacing for manufacturing purposes, they may not be enough for electromagnetic purposes.

Consider two traces that are parallel to each other. If the differential signal in one trace has a greater amplitude than the other trace, it can positively affect the other trace. Then, the signal in the “victim” trajectory will begin to mimic the characteristics of the aggressor’s trajectory, instead of conducting its own signal. When this happens, crosstalk will occur.

Crosstalk is generally considered to occur between two parallel tracks adjacent to each other on the same layer. However, crosstalk is more likely to occur between two parallel traces adjacent to each other on adjacent layers. This is called broadside coupling and is more likely to happen because two adjacent signal layers are separated by a very small amount of core thickness. The thickness can be 4 mils (0.1 mm), sometimes less than the spacing between two traces on the same layer.

The trace spacing to eliminate crosstalk is usually greater than the conventional trace spacing requirements

Eliminate the possibility of crosstalk in the design
Fortunately, you are not at the mercy of cross talk. By designing the circuit board to minimize crosstalk, you can avoid these problems. The following are some design techniques that can help you eliminate the possibility of crosstalk on the circuit board:

Keep as much distance as possible between the differential pair and other signal routing. The rule of thumb is gap = 3 times the trace width.

Keep the largest possible difference between clock routing and other signal routing. The same gap = 3 times the rule of thumb for trace width also applies here.

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

Asynchronous signals (such as RESET, INTERRUPT, etc.) should be far away from the bus and have high-speed signals. They can be routed next to the on or off or power up signals, because these signals are rarely used during normal operation of the circuit board.

Ensuring that two adjacent signal layers alternate with each other in the circuit board stack will alternate horizontal and vertical routing directions. This will reduce the possibility of broadside coupling, as the traces are not allowed to extend parallel on top of each other.

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

Your PCB design tools and third-party applications can help you eliminate crosstalk

How your design software can help you eliminate crosstalk in high-speed PCB design
The PCB design tool has many built-in features that can help you avoid crosstalk in your design. By specifying routing directions and creating microstrip stacks, the board layer rules will help you avoid broadside coupling. Using network-type rules, you will be able to assign larger tracking intervals to groups of networks that are more susceptible to crosstalk. Differential pair routers route differential pairs as actual pairs instead of routing them individually. This will maintain the required spacing between the differential pair traces and other networks to avoid crosstalk.

In addition to the built-in functions of PCB design software, there are other tools that can help you eliminate crosstalk in high-speed PCB design. There are different crosstalk calculators to help you determine the correct trace width and spacing for routing. There is also a signal integrity simulator to analyze whether your design has potential crosstalk issues.

If allowed to happen, crosstalk may be a big problem on printed circuit boards. Now that you know what to look for, you will be ready to prevent crosstalk from happening. The design techniques we discuss here and the features of PCB design software will help you create crosstalk-free designs.