EMI Shielding Techniques in RF Circuit Boards
EMI shielding techniques protect devices from electromagnetic fields, radiofrequency interferences, and electrostatic fields.
Nested shields prevent interference between different components.
A via fence reduces crosstalk and EMI in RF circuits.
EMI shielding enclosures
Any electronic device you design must be compliant with EMI standards set by regulatory boards like CISPR, FCC, or IEC. So, how do designers ensure that EMI standards are met?
Creating products that meet EMI standards begins during the design phase. During this process, various measures—such as the proper placement of components, avoiding sharp bends, and grounding planes—are put in place to reduce EMI. In the final stages of product development, EMI testing is carried out to check whether the EMI of the product is within acceptable limits.
To ensure EMI standards are met, different EMI shielding techniques can be used. In this article, we will discuss some common EMI shielding techniques and future trends in the field.
EMI Shielding Techniques
EMI shielding techniques make sensitive electronic products EMI compatible. EMI shielding protects devices from electromagnetic fields, radiofrequency interference, and electrostatic fields. The signals from a device are protected from interfering with external electromagnetic signals. EMI shielding also prevents the leakage of signals from the device.
How Do EMI Shielding Techniques Work?
EMI shielding minimizes EMI by blocking electromagnetic fields with conductive or magnetic enclosures. The conductive and magnetic materials form a barrier for the electromagnetic fields and they fail to interfere with other devices or signals.
Usually, EMI shielding is grounded so that it provides a path for the internal and external fields and noises that enter the metal enclosure. The grounding provides a low impedance path for the fields and noise currents in EMI shielding.
Using a Faraday cage is the oldest EMI shielding technique used in electronic circuits. This technique is also called volumetric shielding, where a metallic screen surrounds the sensitive circuit on all sides. This technique is also used to shield cables that transmit signals. The metallic screen absorbs the radiated or conducted EMI and prevents it from reaching sensitive electronics. Volumetric shielding is expensive, as poor air circulation inside the enclosure demands excellent thermal management.
Let’s take a look at some of the growing trends in EMI shielding.
Trends in EMI Shielding
When a product or circuit is nested inside several levels of shields, it forms a nested shielding. This prevents interference between different components. Each level of shield may or may not be a Faraday cage. The material used for building each level differs.
This method of EMI shielding is more cost-effective than a single shield made of one material and the device’s EMI compatibility is greater with nested shielding.
EMI shielding in PCBs functions to isolate ICs, passive components, and active components from interfering with each other. All this produces EMI, noise, and crosstalk. The parallel traces in PCBs are mostly the source generating EMI. As the spacing between parallel traces decreases, the effect of EMI increases and results in signal degradation.
The routing of traces as differential pairs is one EMI shielding technique used in PCBs. Another technique is to include a grounded trace between two signal traces. The grounded trace, otherwise known as guard trace, can reduce the cross-talk between two common-mode or differential pairs of signal traces. Guard traces are mostly used to reduce crosstalk in analog and digital traces.
Via fences are constructed in PCB boards to shield microstrip and stripline transmission lines. The via fence is formed by placing multiple rows of vias along the signal trace. The via fence, also called via shielding, reduces crosstalk and EMI in RF circuits, especially the ones with antennas. These are also used to shield ICs, components, and circuit blocks from each other.
Use EMI Shielding Techniques to Increase EMI Compatibility in Circuits
When designing densely routed PCBs, incorporating EMI shielding techniques such as nested shielding, guard traces, or via shielding can help increase the EMI compatibility of a circuit. Cadence’s software offers PCB system design tools that can help the designer to implement EMI shielding in circuit boards.
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