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, 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 on your PCB layout can be disastrous. If not corrected, crosstalk can cause your finished circuit board to either not work at all, or it may be plagued by intermittent problems. Let’s take a look at what crosstalk is and what you can do to prevent it.
What is crosstalk in high speed PCB designing?
Crosstalk is the unintentional electromagnetic coupling between traces on a PCB. 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 is tight. Even though the traces may be maintaining the minimum spacing for manufacturing purposes, it may not be enough for electromagnetic purposes.
Consider two traces 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 characteristics of the aggressor trace instead of conducting its own signal. When this happens, you have crosstalk.
Crosstalk 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 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 spacing between two traces on the same layer.
Removing the potential for crosstalk from your design
Fortunately you are not at the mercy of crosstalk. By designing your board to minimize the potential of crosstalk situations, you can avoid these problems. Here are some design techniques that will help you to eliminate the possibility of crosstalk on your board:
Keep as large of a distance as possible between differential pairs and other signal routing. The rule of thumb is gap = 3 times the trace width.
Keep as large of a difference as possible between clock routing and other signal routing. The same gap = 3 times the trace width rule of thumb works here as well.
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.
Asynchronous signals (like RESET, INTERRUPT, etc.) should be routed away from busses 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 board.
Make sure that two signal layers adjacent to each other in the board stackup 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.
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, it will also provide the required return path for the signal layers.
How your design software helps you to eliminate crosstalk in high speed PCB designing
PCB design tools have a lot of functionality built into them to help you to avoid crosstalk in your designs. Board layer rules will help you to avoid broadside coupling by specifying routing directions, and creating microstrip stackups. With net class rules, you will be able to assign greater trace spacing to groups of nets that are more susceptible to crosstalk. 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 PCB design software, there are also other tools that can help you to eliminate crosstalk in high speed PCB designing 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 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 from happening. The design tips that we have discussed here along with the functionality in your 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 can help you to work through crosstalk and other signal integrity issues in your PCB design? Talk to an expert at Altium.