Mitigate Signal Reflections in High-Speed PCBs

Signal reflections are a common cause of interference between different traces on a PCB. These reflections can vary the logic conditions of a signal, introduce jitter or ringing and increase the frequency of signal distortions, leading to poor performance. In addition, signal reflections can increase the level of electromagnetic radiation and crosstalk between traces, leading to problems with EMI/EMC.

PCBs that are designed for high-speed digital signals must be free of these signal reflections to maintain the desired level of signal integrity and performance. To mitigate these reflections, designers must consider a number of factors, including impedance matching, proper termination and careful layout techniques. In this two-part series, we’ll discuss the effects of reflections and how to minimize them through effective PCB design techniques.

Ideally, pcb traces would be perfectly linear and match the characteristic impedance of the connected components. This would ensure that all the energy of a signal is transmitted without any reflections. However, the reality is that reflections are unavoidable, particularly as a trace length increases. The most significant source of reflections is an impedance mismatch between a signal line and its connected component. The reflections caused by this mismatch create ringing in the PCB’s interconnects, and they can also cause other issues like overshoots and undershoots that degrade signal integrity.

How to Mitigate Signal Reflections in High-Speed PCBs

In order to avoid these problems, it’s important to use good termination techniques like parallel, series, or Thevenin. These termination methods reduce the amount of energy reflected, and they can also be used to eliminate differential mode reflections. However, it’s important to note that termination methods can impact the drive current and may result in trace stubs.

Another way to minimize signal reflections is to carefully consider the layout of the traces and the layer stack-up. Traces should be as straight as possible, with 45deg arcs instead of right angles if they’re curved. Signals should be separated by a sufficient distance to prevent crosstalk, and they should be placed close to the ground plane to further reduce noise. The placement of vias is also critical. Avoid placing vias on the same layer as signal traces, and avoid putting them in close proximity to each other. This can lead to loop areas, which can produce variations in impedance and reflections.

The manufacturing process of PCBs is a fascinating blend of precision engineering and advanced technology. It typically begins with the design phase, where engineers use specialized software to create a blueprint of the PCB layout, specifying the placement of components and the routing of traces. Once the design is finalized, it undergoes a series of checks to ensure compatibility and functionality.

Finally, it’s important to conduct simulations and checks throughout the design process. Failure to do so can lead to expensive re-work and potentially a faulty product for your end-user. Sierra Circuits’ CAD tools make it easy to conduct simulations and checks throughout the design and fabrication process, minimizing signal reflections and other errors before they can cause any major problems. Learn more about our controlled impedance capabilities and how we can help you improve your high-speed PCBs.