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EMI RFI Shielding Review

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EMI RFI Shielding Review

RFI (Radio Frequency Interference), which is also known as EMI (Electromagnetic Interference) is any electromagnetic radiation or disturbance which is emitted from an electrical circuit or electronic component that interrupts or interferes with the operation of other electronic components, or electrical circuits. RFI or EMI may be introduced intentionally as in the case of electronic warfare, or unintentionally from an electronic or electrical device or system. EMI may also be created as a result of a nuclear explosion.

EMI / RFI shielding should be an integral consideration to electronic device or electronics system design. Good RFI / EMI design techniques include, mechanical or electromagnetic shielding, careful circuit filtering design, adequate circuit ground design, as well as, attenuation to the bonding of the ground points.

The following are electro-mechanical design considerations for achieving EMI / RFI performance requirements. The following is by no means - exhaustive. Engineers Edge does recommend mechanical and electronics engineering and design staff to work closely with each other to tackle EMI RFI challenges early in the design process.

Packaging / Enclosure Design

Select enclosure materials which absorb or block EMI / RFI. Consider metal shielding barriers with discontinuities. Within your electronic enclosure, allow for the installation of a EMI RFI gasket device or materials. There a many types and configuration of gaskets or elastomers which help facilitate EMI sealing of your electronics enclosure. As required, design separate isolated areas within your electronics enclosure to separate / isolate noisy or sensitive electronics circuits and components. The separated electronics maybe connected high impedance feed-through's.

PCB Design
When designing a PCB, the design goal is to control the following:

  • EMI RFI from the PCB circuitry.
  • Sensitiveness of the PCB circuits to EMI RFI interference.
  • Coupling between PCB circuits and external electronics systems.
  • Coupling between circuits on the PCB.

PCB layout designers should minimize impedance discontinuities by use of low amplitude signals. If you have clock frequencies above 10 MHz, it may be necessary to design a multi layer board with a ground layer. If this is not possible or is cost prohibitive for your product, try to use guard banding (grounds on each side of signal traces). Separate components such that noisy and sensitive circuits are placed as far away from each other as possible. Keep clock traces, buses and chip enables separate from I/O lines and connectors. Orient clock trace runs perpendicular to signal trace runs, and design the smallest possible trace width. If your clock devices are off the PCB, locate them close to the system connectors. If your clock devices are on the PC board, locate them such that they are central to the connecting components with the objective to minimize onboard distribution traces. Locate your input / output chips near the associated connectors. Consider using a resistor, inductor, or ferrite bead device to dampen the output circuits. Circuit types (i.e., digital, analog, power) should be separated, as well as their grounds.

Cabling and Interconnects

When cables are required to connect assemblies and PCBs, the lengths should be minimized. Long service loops can be very disruptive. If PCBs are properly designed, the requirement for shielding of internal cabling will be minimized. However, if it is found that cable shielding is required, the technique used to ground the shield is critical to the attenuation afforded by the shield. Cable shields should not be used as signal returns. Consider using wire rib bins where practical.

Input Output Filters

Often to suppress power line and signal line EMI RFI, some form of filtering is required. Filter attenuation is highly dependent upon source and load Some knowledge of basic filter design is helpful in selecting which filter type to try first. Where common mode filtering is required, line-to-ground capacitors and common core inductors should be used. Where differential mode filtering is required, line to line capacitors and discrete series inductors should be used.

Most filter manufacturers, given some knowledge of a particular device and the EMI problem, can assist in selecting a suitable filter. The only way to be sure that a filter will reduce EMI to compliant levels is to test the equipment for conducted emissions, and be prepared to try several different filters. This trial-and-error approach may be unscientific, but in most cases proves to be the fastest, most cost effective, and minimum risk approach.

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