Remember back in school when the teacher would ask the class a question and you actually knew the answer! You and a few other students would excitedly raise your hands hoping the teacher would call on you. And if you were lucky enough to speak first you felt vindicated for the time you invested studying while some of your friends opted for other activities like playing outside. Although being able to speak without interruption was elating, the teacher’s motivation was to eliminate the interference of several people speaking at once.
A similar situation exists on your PCB where multiple signals on different traces can interfere with each other. When signals create electromagnetic energy that is transferred to parallel traces it is known as coupling. Coupling between conductors can be both good and bad and is expressed as a ratio between 0 and 1 (or 0% and 100%). For example, bundled conductors or twisted cables can be used to achieve near perfect coupling ratios that support good signal transmission. Bad coupling or crosstalk usually refers to interference on one conductor due to another adjacent conductor that distorts or reduces signal quality.
Let’s take a look at the causes of crosstalk and then how Altium Designer® can help you minimize the effects of crosstalk on your board.
What Causes Crosstalk?
Crosstalk is one of the signal integrity problems that PCB designers have to contend with. Typically, crosstalk occurs when two different signal propagation paths are too close to each other and the EM fields that naturally occur extend beyond the space that separates the conductors. As this interference is unwanted, it can be classified as noise and distorts or degrades the purity of the signal resulting in crosstalk.
As electronics products become increasingly smaller, the PCBs of which they are comprised must also be smaller. However, these dimensional reductions are typically paired with demands for greater functionality. To achieve these objectives, which may seem to be opposed to each other, component packages are smaller but have high numbers of connections or pin counts. Obviously, this leads to a tighter pitch between pins that promote capacitive coupling. Coupling between pins may occur for connections that extend from the component along the board surface or from underneath for signal flow through vias.
As traces take up more real estate on your board, there are more areas where crosstalk can occur. This includes at pads or the start and end of trace paths, along with adjacent routes on the same layer and between layers. Crosstalk at pads is similar to what occurs between component pins. Along with traces, the interference is typically due to similar length signal paths with inadequate separation at the same angle or a single trace with an acute angle bend. When signal layers in your stackup are not sufficiently isolated by dielectrics crosstalk may occur between traces on different layers, as well.
Crosstalk Elimination Techniques in Altium Designer
It may seem that crosstalk is an insurmountable obstacle to good signal integrity for PCB design, especially for small, densely packed, complex boards. Certainly, it is a significant design challenge yet there are techniques that can be employed to reduce the effects of crosstalk on your circuitry’s operation. In order to make the best use of these we first need be able to analyze our board’s signal to determine what, if any, crosstalk elimination is needed. Altium Designer provides just such a tool for performing Signal Integrity Analysis, shown below.
Signal Integrity Analysis example
Signal integrity analysis in Altium includes the ability to check signal rise times, fall times, termination and crosstalk. You also have the ability to define models and set rules and constraints and other setup for the analyses. Once a crosstalk problem is confirmed, it should be addressed by modifying your trace routing as necessary for the same layer or adjacent layers as described below.
Ensure Adequate Spacing Between Traces
In most cases, the cause of crosstalk is inadequate spacing between the coupled conductors. Consequently, the best solution or technique for reduction is to increase the spacing. In Altium Designer, this can be done quite easily by using highlights, which allows you to move single elements and traces or groups at once. When making changes to your traces be sure to follow good PCB layout routing tips.
Using net classes to set trace spacing
Minimize Parallel Trace Lengths
At times, such as for differential pairs, matching your signal paths is preferable. In these cases, ensuring that copper weights, trace widths and lengths are the same for both signals is the design objective. By contrast, when parallel traces are for different signals the goal is to minimize their interaction or coupling. If adequate spacing is not an option, you can run one trace longer than the other or run one trace perpendicular to the other to reduce crosstalk.
Using Ground Planes to Separate Signal Layers
Although less common than same layer crosstalk, interference between adjacent layers in the PCB stackup does occur. The primary cause of this type of coupling is a lack of isolation of the signals due to insufficient insulation between the layers. The source of insulation between adjacent layers is the dielectric material upon which copper layers are etched. In Altium Designer, it is a simple task to design your stackup using the Layer Stack Manager dialog, shown in the figure below, where you can select material type, set the thickness and define the dielectric constant.
Using Layer Stack Manager to define PCB stackup
As an alternative to using different materials for the stackup, ground planes can be inserted between signal layers. This serves the purpose of increasing isolation and may be useful for providing shorter grounds for your design.
Crosstalk is a signal integrity problem that can affect your PCB layout at almost any point where two conductors are near to each other. The best crosstalk elimination techniques are those that either insert space or barriers to reduce the coupling. Before any of these can be applied, crosstalk must first be identified. Altium Designer provides a multi-functional signal analysis tool that allows you to diagnose your board’s signals and determine the best elimination technique(s) to apply. If you would like to test drive Altium Designer, you can get a free trial here.
For more information on crosstalk elimination techniques that you can incorporate into your PCB design, talk with an Altium PCB design expert.
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