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Faraday Cage

1) The document investigates whether using "picket fences" of ground vias around the edges of a PCB can act as an effective and low-cost Faraday cage. 2) An EM simulation was conducted comparing fields around a PCB with and without ground vias, finding only a small difference of 0.5dB attenuation with vias. 3) Simulated emissions at 10 meters also showed little difference between PCBs with and without ground vias. 4) The conclusion is that while picket fences provide some minor attenuation, they do not significantly reduce emissions and are not an effective substitute for other EMC techniques given the board space and effort used.

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Hugo Damn
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725 views3 pages

Faraday Cage

1) The document investigates whether using "picket fences" of ground vias around the edges of a PCB can act as an effective and low-cost Faraday cage. 2) An EM simulation was conducted comparing fields around a PCB with and without ground vias, finding only a small difference of 0.5dB attenuation with vias. 3) Simulated emissions at 10 meters also showed little difference between PCBs with and without ground vias. 4) The conclusion is that while picket fences provide some minor attenuation, they do not significantly reduce emissions and are not an effective substitute for other EMC techniques given the board space and effort used.

Uploaded by

Hugo Damn
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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Picket Fences: A poor mans faraday cage?

Karthik Raj Guruchandran www.basebandhub.com Abstract:


Faraday cage is an enclosure that shields electromagnetic fields from penetrating into or radiate out of the enclosure.Faradaycageshavebeenaroundforalongtimeandaregenerallyusedtoisolatetestequipmentsorto measureradiatedemissionsandimmunityfromaDUT.TheconceptofFaradaycageworkssowellthatengineers havebeenusingitonPCBdesigns.Butsomeengineersgotoanextremeofusingpicketfences(groundviason PCBedges)andbelievethattheyactasafaradaycagewhilebeingcosteffective.Canthisbetrue?Isetoutto investigate.

AFaradayCageonPCBs:
ThepopularwayofusingfaradaycageonPCBsisby constructing shields. When a particular area in the PCB needs to be shielded from incoming radiations or from outgoing radiations, shields are used. As miniaturization got popular, the number of shields usedinaPCBhasincreasedsignificantly.Theshields onPCBsmimicfaradaycagesandaregenerallyused to isolate electric fields and not magnetic fields andtheyworkquitewellincontainingthefields.But in projects where cost is involved, engineers recommend the usage of stitching ground vias around the corner of the PCB to help shield emissions. Usage of shields is an accepted practice, but the usage of vias around the corner of the PCB to try andcontainemissionsfromaPCBisverydebatable. Even very experienced engineers have different thoughtsonthispractice.Theusageofviasaround the board edge however is not something that forms a faraday cage (unless there are supporting mechanics involved that have a conductive metal frameontopandareconnectedtothegroundlayer in the PCB or the whole system). Though some engineers completely dismiss this approach, there areafewwhoadvocatethis. So,IwantedtotryandbuildasimplePCBmodelin CST a 3D EM Simulation tool and wanted to investigate the usage of picket fences (or placing vias at regular intervals around the PCB) and evaluateEMCperformancefrombothnearfieldand a far field point of view. I constructed a simple microstrip model with 50 ohm trace impedance. In one of the configurations, I added vias that stitch the top ground plane to the bottom ground plane andontheotherIdidnthaveany.

Figure:Picketfences(Groundviasaroundtheboardedge)
www.basebandhub.com

Figure:Setupwithoutgroundvias(Left)Setupwithgroundvias(right)
Ithenmeasuredthe3DfieldsaroundthePCB(and not the trace itself) to see if the picket fence plays anyroleinthereductionofemissionsfromthePCB. The scales on the top right corner show that the fieldjustoutsidethePCBisabithigherwhenthere is no faraday cage. The difference in values is very less and equates to around 0.5dB. A similar performance can be seen at 1GHz frequencies as well.


Figure:Emissionsat200MHzwithoutgroundvias(Left)withgroundvias(right)


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Figure:Emissionsat1000MHzwithoutgroundvias(Left)withgroundvias(right)

EMCPerformance:
Now that we are clear that the field attenuated by using picket fences is around 0.5dB, I felt it is probablyrelevanttounderstandtheemissionsfrom the PCB at 10 metres (to mimic a standard EMC measurement from a PCB). The result of Emissions at 10 metres was very similar to each other.


Figure:Emissionsat10metredistancefromthePCB

Conclusion:
It is very clear from the above results that picket fences do not help in reducing emissions in both near and far fields (the computed far field is an extrapolated value based on the near fields). Thoughithassomebenefits(attenuationofaround 0.5 dB), it does not justify the amount of board space and effort required. It is far better to concentrate on reducing emissions using other techniques than to try and shield them with picket fences.

www.basebandhub.com

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