Minimize the susceptibility of noise sensitive circuits in seconds with electromagnetic shielding
NEW! Check out our video "How to measure effectiveness of a shield early in the design cycle before the anechoic chamber test". Click here for the video.
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How Does Shielding Work?
When an incident electric field traveling in the air hits a metal surface, the metal causes the penetrating field strength to decrease. The metal causes the field to be replaced with conduction currents that flow in the metal close to the surface. A very small (exponential decay) amount of the field does pass through, but for emissions, this is never a problem. The metal chassis serves as a shield. The field from all the radiating surfaces inside it are blocked and kept inside the box, with the only noise coming from the cables or wires that enter or exit the box and from holes made in the box.
If a metal enclosure is to be used, its shielding effect should be utilized. However, it is always better to reduce the noise inside the box than rely on shielding.
Grounding the Shield
The shield has the difficult job of providing a terminating or conducting surface for direct ESD hits, ambient fields, and internal fields, as well as noise carried on the cables entering and exiting the chassis. To do this well, the shield should be thought of as an RF conducting plane, with the least number of breaks and impedance's between the source of the RF currents and the ground reference point.
Cables and Bypassing to the Shield
The PCB should have a large separate ground area tied to the enclosure shield which serves as the ground for the I/O bypassing capacitors. These capacitors provide final filtering of system noise, but also are intended to filter noise picked on the cable outside the box. An RF path is required as the connection to the chassis.
How Can You Measure the Effectiveness of Shielding in Real-Time?
The use of component shielding and shielded printed circuit board (PCB) traces can avoid unpleasant problems late in a product design cycle. EMI shielding is one of the ways to minimize susceptibility to radiated spurious signals in noise sensitive circuits. Sometimes, radiating components must be located on both sides of the PCB, which may then necessitate shielding on both sides of the board as well. This will lead to a more costly, complicated board assembly process.
Visualization is the key to better understanding of a PCB's electromagnetic performance. EMxpert shows the impact of the EMI shielding by board location and frequency. It also measures the effectiveness of EMI shielding by mapping near-field emissions generated by current flow on the surface of your board and helping diagnose any problems in real-time. The spatial comparison tool then facilitates immediate comparison between the shielded and unshielded solutions.
Below example shows the emissions on the PCB before implementing the shield:
Below example shows the emissions on the PCB after implementing the shield: