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How to Suppress EMI/RFI in High-Speed PCB Designs

The effects of electromagnetic interference (EMI) and radio frequency interference (RFI) are particularly troublesome when designing PCBs for high-speed systems. PCB components can be digital and analog with transmission lines between them, using etched traces of copper on the PCB in lieu of discrete wires or cables. As frequency increases in these PCB designs and signals are enhanced, noise related to those frequencies, unfortunately, also get enhanced, thus creating EMI and RFI.

Successfully reducing EMI/RFI calls for careful preparation, planning, and close interaction between OEM and electronic manufacturing service (EMS) provider. In this instance, it is a good idea for OEMs to get a good handle on suppressing EMI/RFI in their projects. The following design guidelines help in this regard.

  1. Assure that differential pair tolerances are at 5 percent.
  2. Layer construction during PCB design layout and fabrication plays a vital role in reducing EMI/RFI. Symmetrical strip line geometry is the best approach in minimizing or eliminating EMI/RFI. A single signal layer is sandwiched between two ground layers with the ground planes absorbing virtually all the noise and crosstalk.
  3. Properly shielding clock circuitry is critical as a major step in reducing EMI/RFI.
  4. Component placement in a mixed signal design requires the analog section be totally isolated as far as placement, routing, and plane separation.
  5. Analog traces should only run underneath their analog reference power or ground plane. Conversely, digital traces should run under the digital section with respective power and ground planes. Impedance is thus kept constant, and there is a good return path for signals.
  6. When routing RF circuits, it is important to avoid extreme bends at 45∫ angles.
  7. Via stitching is another technique for suppressing noise and crosstalk. Rather than a single ground plane on PCB layer, this technique implements multiple ground planes evenly distributed throughout the PCB.

By carefully and correctly implementing these design steps, EMI/RFI can be reduced 90 percent or more on a PCB design. Omitting or giving little attention to any one of these design measures can boost EMI/RFI to increase noise and cross-talk levels well beyond a design’s specifications, thus impeding proper signal transmission and reception.