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The Synergy of Filtering and Shielding

Introduction

In previous Product Insights articles (references 1 through 7), we’ve primarily addressed filtering and shielding topics separately. While the two are important foundational topics, it’s time to take a deeper look into our roles as product developers toward the goal of achieving the most robust (lowest EMC emissions, high EMC immunity) product design possible at the highest frequency of concern in the system we’re developing.

This article briefly describes the concept of the synergy of filtering and shielding and why knowing it is important.

Origin of the Synergy of Filtering and Shielding Concept

I’ve picked up on the concept of the synergy of filtering and shielding from world-renowned and UK-based EMC expert Keith Armstrong. Keith describes the synergy of filtering (and much more) in two of his books on EMC (references 8 and 9).

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VSWR and its Effects on Power Amplifiers

Voltage Standing Wave Ratio results from an impedance mismatch between a source (an amplifier) and a load (test application). This mismatch can influence the performance of the source.

Pro-Tip: If you don’t already have Keith’s books, you should seriously consider picking them up as soon as possible. They are chock full of very useful (and practical) EMC design concepts and are among the most useful books on EMC I’ve read.

The Essence of the Synergy of Filtering and Shielding

Mains filters and the like provide very little attenuation above ~30 MHz (highest frequency of the conducted emissions test) and permit radio-frequency (RF) emissions to emanate from accidental antennas (i.e., cables) attached to your device to cause both EMC emissions and immunity problems. Because the low-frequency (LF) filtering offered in this situation is less than ideal and much lower than required for the overall high-frequency (HF) shielded enclosure or shielding-can (if located on a printed circuit board), overall shielding effectiveness (SE) is severely degraded.

All unshielded cables that enter/exit the enclosure (mains included) must be filtered with good attenuation at the highest frequency of concern, if good HF shielding is required (often the case in today’s environment of high-speed, high-frequency products and regulatory compliance requirements). In short, the synergy of filtering and shielding means that filter attenuation versus frequency should match desired shielding performance.

How the Synergy of Filtering and Shielding is Accomplished

The synergy of filtering and shielding is best accomplished by what can be regarded as the belt and suspenders approach to EMC. Briefly, here are some ideas to consider:

  • Utilize mains filters that specify their attenuation much greater than the typical 30 MHz, such as those filters specified as meeting military EMC requirements.
  • Add additional HF filtering where needed.
  • Utilize traditional high-performance, three-terminal feedthrough capacitors.
    • Use 360° electrical bond so internal and external surface currents stay separated of either side of the shield.
  • For high-volume manufacturing utilizing shielding-cans on a PCB, use a surface-mount device (SMD) 3-terminal capacitor filters with the center terminal soldered to a guard trace with the filter’s input and output terminals placed on opposite sides of the shielding-can’s wall. Holes in the can straddle the SMD capacitors roughly at their center-point, one terminal of the SMD capacitor is located inside the shielding-can and the other terminal located on the outside. This is not as good as a 360° connection offered by using traditional types of feedthrough capacitors but is the best we can do in this situation.
  • Utilize a dirty box/clean box method for mounting filters in the walls of shielded enclosures (see reference 8 for examples). This design technique helps isolate the noisy, bad RF signals from the good, clean ones. Along these same lines,
  • Carefully route filter inputs and outputs, keeping the noisy input side wires or traces far away from the clean output wires or traces.
  • Filter all unshielded signal or power cables entering/exiting the shield enclosure using 360°
  • Other design techniques may be appropriate depending on the unique product design and can be developed using your own imagination and your own design for EMC prowess.

Summary

And there you have it, a quick run-through of the concept of the synergy of filtering and shielding and why it’s an important concept to keep in mind. For more information, please see the following:

References and Further Reading

  1. In Compliance Magazine. (2021, November 8). How EMI Filters Are Specified for RF Performance.
  2. In Compliance Magazine. (2020, November 5). An Alternative Approach to Specifying an EMI Filter.
  3. In Compliance Magazine. (2020, July 31). How EMC Shielding is Defeated.
  4. In Compliance Magazine. (2019, November 12). Let’s Talk About Why Filters Fail.
  5. In Compliance Magazine. (2019, August 6). Let’s Talk About Shielding at the PCB Level
  6. In Compliance Magazine. (2018, November 12). What Every Electronics Engineer Needs to Know About: Filters
  7. In Compliance Magazine. (2018, August 2). What Every Electronics Engineer Needs to Know About: Shielding
  8. Armstrong, K., EMC Design Techniques for Electronic Engineers, Armstrong/Nutwood UK, 2010.
  9. Armstrong, K., EMC for Printed Circuit Boards, Armstrong/Nutwood UK, 2010.

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