How to Reduce Crosstalk in Your PCB Layout
Have you ever been in a meeting where there were multiple conversations going on at the same time? This kind of scenario often means that the meeting isn’t as productive as it could be, plus the information that you need may get drowned out by the other conversations. You will probably notice that people start leaning into each other to hear better while raising their voices at the same time in order to be heard. Crosstalk like this can create a real mess and is one of the reasons why people will look for excuses to avoid meetings.
Although getting out of a meeting may not be the worst idea in the world, missing out on important information for your job is. In the same way, when there is crosstalk on your circuit board, the board may not work correctly, and you could lose important information there too. To avoid this it is in the best interest of PCB designer to find ways to eliminate the potential of crosstalk in their designs. Let’s talk a little about crosstalk and some different design techniques that can answer how to reduce crosstalk in your PCB layout.
Crosstalk on a Printed Circuit Board
Too much activity in a circuit board can make the transmission of a signal difficult. Think of two traces on a circuit board running together side-by-side. If the signal of one trace is “louder” with more amplitude than the signal of the other, it could over-power that other trace. And just as it can be difficult in a noisy room to keep your own thoughts straight when trying to speak, the “victim” trace on a PCB will be similarly influenced by the louder signal. The problem then is that the victim signal will start behaving like the aggressor signal instead of behaving the way that it is supposed to.
Crosstalk is defined as the unintentional electromagnetic coupling between traces on a printed circuit board. The overpowering of one signal in a trace by another one is the result of this coupling even though the two traces are not in physical contact with each other. This can happen on a PCB that has acceptable trace spacing for manufacturing but where the spacing is not acceptable for crosstalk.
In addition to the potential for crosstalk between two traces that are side-by-side on the same layer, there is an even greater risk for traces that are running parallel to each other vertically between two layers. This effect is known as broadside coupling and happens because the two signal layers are separated by only a very small thickness of core material. This distance is often less than the spacing between two traces on the same layer.
Figure out your trace layout through strong routing guidelines
How Your PCB Design Tools Can Help You to Work Through Crosstalk Issues
You’ve got a great ally with you these days when working to reduce and eliminate areas of crosstalk on your circuit board, and that is the functionality in your PCB design tools. At one point, design tools were very minimal in what they could help you with, but that is no longer the case.
You can set up a vast array of design rules to specify clearances between traces and from traces to other objects on the board. You can even set up different values of clearances depending on specific nets or areas where those nets are being routed through. This will help you immensely to set your design up so as to avoid situations where crosstalk could potentially develop.
Design tools also have specific functions for routing differential pairs at specific widths and clearances, and you can set up trace lengths as well as rules to match specific trace lengths to each other. You also have the ability to specify which layers of the board certain nets can be routed on and what the preferred direction of traces on those layers should be. There are also crosstalk calculators available to you as well as other simulation and analysis tools. The design tools that we have available to us today contain all kinds of design constraint features that can help with problems like crosstalk, we just have to put them to work.
Let your testing be a bit more steady than this
Design Methodology for How to Reduce Crosstalk
Now that we’ve talked a little about what crosstalk is and how your PCB design tools can help you, let’s look at a couple of basic PCB design tips for avoiding potential areas of crosstalk on your design:
- Configure your board layers so that two adjacent signal layers will have preferred routing directions that cross each other instead of running parallel to each other. If layer two is running “north to south,” then make sure that layer three is running “east to west.” In this way you can minimize the possibility of broadside coupling.
- Use ground planes between two adjacent signal layers to reduce the chance of broadside coupling even more. Not only will this increase the distance between the layers but also this configuration will give you a much better return path through the ground plane.
- Keep as much space between high speed routing (differential pairs, clock routing, etc.) and other routing. The general principle here is by spacing out traces at three times their line width, measured center to center, 70% of their electrical field can be stopped from mutual interference.
Crosstalk can cause serious problems in your design and you will want to learn as much about it as possible. What we’ve given you here is a great first step in that direction to get you on the right path. Another step in the right direction is to use PCB design tools that have been created to help you as much as possible to avoid problems like crosstalk in your PCB designs. Cadence’s PCB editors have a vast array of design rules that you can use to set up your design to reduce crosstalk in your PCB layout.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.