One of the sneakiest and trickiest problems that printed circuit boards might encounter is crosstalk. The worst part is that it frequently happens sporadically or in a way that is difficult to duplicate in the last stages of a project, where it normally only occurs. Therefore, it is crucial for every electrical designer to quickly rule out all potential causes of crosstalk on a PCB. Clock signal, periodic and control signals, data transmission lines, and I/Os are all negatively impacted by crosstalk. Crosstalk can result in current and voltage levels that are higher than the normal thresholds of logic devices, creating “false” logic states that can have an impact on the performance of many circuits. Analog signals might suffer from crosstalk because it introduces extra noise. We’ll go into more depth about crosstalk in the next paragraphs, as well as how to assess it and get rid of it if the circuit employs high frequency signals.
PCB crosstalk
Crosstalk is the name given to the unintended electromagnetic connection that occurs between a printed circuit board’s traces. Without the two traces being in direct touch with one another, an excessive voltage or current on one trace might have undesirable consequences on the other. When traces are not appropriately spaced apart, this behaviour frequently happens on PCBs. It is fairly easy to explain the phenomena. An electromagnetic field of a particular strength is always produced when a conductor is traversed by a passage of electrical charges. The possibility that a signal may couple with a neighbouring signal rises as the signal’s speed (frequency) increases. In more technical terms, inductive (or magnetic) coupling and capacitive (or electrical) coupling are the two forms of coupling.
A magnetic field is produced when current travels through a conductor, such a PCB trace. According to Faraday’s second rule of induction, when this field touches a neighbouring trace, it produces an electromotive force (voltage). When the induced voltage is high enough to jeopardise the integrity of the signal on the same trace, a phenomena known as magnetic or inductive coupling—which is a phenomenon—becomes a concern.
The current flowing through a PCB trace can also produce an electric field in addition to a magnetic field. It creates a capacitive coupling when it reaches a neighbouring trace, which leads to the signal’s integrity being compromised. Another name for this phenomena is parasitic capacity. An eye diagram, one of the most used methods for determining signal integrity, is shown in Figure 1.
Crosstalk can happen between parallel traces belonging to different levels in addition to neighbouring traces put on the same layer. When there is less dielectric material between the two layers, a process called as broadside coupling takes place. This thickness is often smaller than the space between two traces that are put on the same layer and can be as low as 4 mils (0.1mm). As we’ll see in a moment, keeping enough distance between the traces that carry high-speed signals is one technique used to get rid of crosstalk. As a general guideline, leave a space at least three times as wide as the neighbouring traces in between them.
Crosstalk prevention methods
While crosstalk cannot be totally removed, it can be reduced to the point that it has no appreciable impact on the integrity of the signal. After defining crosstalk, we may examine the primary strategies frequently employed during PCB design to lower this risk.
1. Minimum spacing between traces
It is possible to define rules for the PCB layout in the CAD tool being used, such as the minimum distance between two traces and the minimum distance between each trace and the board’s components. Additionally, alternative distance values can be configured in relation to a certain netlist or the region via which the netlist will be routed. Applications for PCB design software frequently provide capabilities that allow users to choose things like the width and separation of differential signal pairs, the PCB layers on which netlists can be routed, and the desired directions that traces can go. There are several calculators available online that can determine the degree of crosstalk for microstrip or stripline traces. The basic guideline to be taken into mind is that as the distance between the traces increases, the coupling—both inductive and capacitive—decreases.
2. Maintain perpendicular traces on neighbouring layers.
In order to avoid their traces being parallel, PCB layers must be set up so that signals crossing neighbouring layers have directions perpendicular to one another. It is also used to indicate that if the traces on one layer run “from north to south,” the traces on the layer directly next to it should go “from east to west.” You may reduce the negative impacts of coupling on the broadside by taking this easy precaution. Figure 2 depicts this strategy graphically, with the layout to use on the right and the one to avoid on the left.
3. Use ground aircraft
Inserting a ground plane between two adjacent signal layers is a smart idea (or, alternatively, a power plane). By doing this, the possibility of broadside couplings happening is significantly decreased. With this technique, the distance between the layers is increased while also improving the return path to ground needed for the signal layers. A traditional four-layer PCB with two exterior signal layers, an interior ground plane (0V) layer, and an internal power supply layer may be shown in Figure 3.
4. Utilize the ground return route
An additional method for reducing crosstalk involves linking the ground return channel with the high frequency signal, taking use of the parallelism between the traces, even if it may seem to go against what has been discussed earlier. In fact, the effects are removed with a corresponding decrease in crosstalk since the ground return channel has the same amplitude but faces the opposite way from the signal.
5. Employ differential signalling
Utilizing differential signals, which are two signal lines with the same amplitude but opposing polarity that combine to create a single high-speed signal, is another technique to preserve signal integrity while limiting the impacts caused by crosstalk. The signal retains a high integrity even in the face of large external noise because, during reception, the signal is acquired as the difference between the voltages of the two signal lines, and since electromagnetic noise affects both lines equally. The recommendation is to keep the differential signal pairs and the other PCB traces as far apart as feasible. As a general guideline, pick a distance that is at least three times the track’s width.
6. narrow the parallel traces’ widths
When it is impossible to eliminate parallelism between traces, it is imperative to make sure that they are as short as possible to minimise entanglement.
7. Remove low-frequency signals from adjacent traces.
Clocks and other high frequency signals ought to be as far away from other signal lines as practicable. The rule of thumb may still be used in this situation, resulting in a minimum distance that is three times the trace width.
8. Distinguish asynchronous signals
Asynchronous signals like reset or interrupt lines must use traces that are as far away from high frequency signals as is practical. As these signals are only utilised during certain phases of the circuit operation and not continually, asynchronous signals are frequently positioned next to power lines or signals that regulate the switching on and off.
Conclusion
When a circuit employs high frequency signals, crosstalk can have severe effects on how well it functions. The electronics industry is now requiring circuits to be smaller and quicker, which has led to PCB traces having less and less room to spread out and being closer together. This feature increases the likelihood that the electromagnetic field created within one trace may interfere with the signal of the other, especially when the traces run parallel to one another. Therefore, it is crucial that the PCB designer use the best approaches to reduce or eliminate the impacts brought on by crosstalk.
