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PCB Design Principles to Reduce Crosstalk

Updated: Nov 25, 2020

The PCB designer is no stranger to EMC issues, especially the issue of crosstalk. Crosstalk is any undesirable signal transfer between communication channels or traces within a PCB. Put simply, crosstalk occurs when one trace's signal overpowers another trace's signal. The aggressor, whose signal is louder or stronger, can keep the victim trace from communicating information properly, causing the board to work incorrectly. In addition, the victim trace will often try to "raise its voice" or strengthen its signal in response, thus acting like the aggressor trace.

Because re-designing PCBs due to noncompliance is frustrating, it is in the designer's best interest to incorporate crosstalk mitigation techniques into the design cycle before submitting the product to a testing laboratory.

The Nitty-Gritty on Crosstalk Crosstalk is near-field EMI and comes with two components: common impedance coupling and electromagnetic field coupling. Common impedance coupling occurs when two or more signals share a common return. Electromagnetic field coupling occurs due to capacitive and/or inductive field coupling between circuits.

Despite the fact that traces placed within a PCB are not physically touching and are often amply spaced, traces running parallel on the same layer or running parallel vertically between two layers are susceptible to crosstalk. This occurs when the aggressor trace, which usually produces an intentional signal, couples through a mechanism and into the victim trace. This coupled signal will arrive when the victim trace is susceptible to noise, allowing the coupled signal to exceed the available noise margin within the victim trace.

Tips for Reducing Crosstalk To reduce crosstalk within a PCB, designers should follow the below steps and consider them in their initial design to eliminate the need for re-design:

  • Use a PCB design tool to determine clearances between traces during the initial design. This should eliminate crosstalk completely.

  • Configure board layers so adjacent signal layers will cross each other rather than run parallel to one another. This should eliminate broadside coupling.

  • Increase the distance between layers by placing ground planes between adjacent signal layers. This should also eliminate broadside coupling.

  • Space traces three times their line width.

  • Separate groups of nets according to signal amplitude. This should minimize the aggressor trace's signal.

  • Change the timing between the aggressor and victim traces so that they do not interact.

  • Reduce the available noise margin.

  • Reduce the number of aggressor traces.

By following the above steps, designers can eliminate crosstalk within their PCBs before or while tinkering with circuits. By eliminating crosstalk before EMC testing, designers can put their products on the market faster.

At Rhein Tech Laboratories, Inc., our engineers can help you successfully meet your new product target launch dates with PCB design reviews and analysis implemented early in the development cycle. In some cases, our early participation can shorten the timeline for the development cycle and reduce your costs. At a minimum, RTL engineers evaluate for the following parameters during PCB design reviews and analysis; however our evaluation is not limited only to the parameters listed below:

Schematic:

  • Termination type

  • I/O Port filtering

  • Isolation of multiple power

  • Decoupling capacitor scheme

PCB Layout:

  • Trace length

  • Number of Via-holes

  • Signal/Ground traces

  • Traces near plane edges

  • Traces crossing G/V planes

  • Estimation of radiated emission fields

Crosstalk

  • Grounding vias along ground-plane edge

  • Filters on a trace connected to a connector

  • Differential pair trace check

  • Discontinuities of return current return path

  • PCB stack-up

  • PCB impedance & resonance

Contact us today for a quote and let Rhein Tech ensure your PCB design is thoughtful and compliant. Sources: https://resources.pcb.cadence.com/blog/2019-how-to-reduce-crosstalk-in-your-pcb-layout, http://www.sigcon.com/Pubs/news/6_01.htm

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