Gentex Shield Tutorial for ME and PCB Designers

1 Shielding TutorialGentex EME LabShielding Course do we need shields? to the Basic Shield Design and Shielding effectiveness of various materials in a Near Field Magnetic Measurement Lab Shield Tutorial3 The Question: Why do we need Shields?EME Lab Shield TutorialWhy do we need shields? Immunity Prevent external energy from interfering with sensitive circuits Emissions Prevent noisy circuits and devices from interfering with neighboring devices Self Compatibility Prevent a device from interfering with itselfEME Lab Shield Tutorial4 Shields: Definition and Misconception5 Definition: A Shield is a conductive barrier enveloping an electrical circuit to prevent time varying Electromagnetic fields from coupling or radiating from the.

2 Most engineers take it as an almost unshakable axiom of engineering faith that a conductive surrounding will provide adequate shielding protection in all shields can be very effective, Designers will get the most performance when some key issues are kept in Lab Shield TutorialWhat makes a good Shield ?EME Lab Shield Tutorial6 It Frequency of interference Type of interference: Magnetic Field Electric Field Location of Field: Near Field Far Field Number of openings and size of openings (Apertures) Type of shielding material (Conductivity/Permeability) Thickness of shielding material Available mating surface (printed circuit board)7 Introduction to the Basic Shield Design ProcessEME Lab Shield Tutorial8 Aperture ConsiderationsEME Lab Shield TutorialAperture Design.

3 91)Holes and slots act as windows for EM radiation to penetrate or escape a shield2)Many small apertures allows less leakage than a single large aperture of the same area3)Models show that in general, the aperture length should not exceed /50 for the highest frequency to be shielded ( Wavelength)EME Lab Shield TutorialAperture Design:101)The main item determining the leakage from a slot is the maximum linear dimension (not area) of the )Remember to take into account the highest frequency harmonic )Multiple apertures farther reduces the shielding effectiveness. The amount of reduction depends on:a)The spacing between the aperturesb)The frequencyc)The number of aperturesEME Lab Shield TutorialAperture Equations:SEdB= 20 Log10( /(2L)), where L< /2 Where:SEdB= shielding effectiveness = wavelengthL = aperture length, longest dimension This is applicable for slots with a dimension equal or less than /2 wavelength.

4 The equation illustrates: The shielding effectiveness is 0 dB when the slot is /2long and Increases 20 dB/decade as the length L is decreased. Reducing the slot length by increases the shielding by 6 Lab Shield TutorialEffect of Aperture Length on Shield Attenuation:12 SEdB= 20 Log10( /(2L)), where L< /2 EME Lab Shield TutorialEffect of Aperture LengthEME Lab Shield Tutorial13 Aperture Length vs. Frequency for Various Attenuations:14 EME Lab Shield TutorialShield Attenuation with Multiple Apertures and fixed and Aperture Length15 RdB = 20log10( /2L) 20log10(n1/2)EME Lab Shield TutorialEME Lab Shield Tutorial16 Affects of Apertures on Shield CurrentsAperture Design and Babinet s theory behind magnetic field shielding provided byinduced currents presumes that currents will flow as long as there are no obstacle in their is essential that any and all apertures be arranged in such a way as to minimize their effect on the have HF resonances, so an induced HF current flowing on the Shield can cause the aperture to act as a transmitting antenna (Babinet Principle or Effect).

5 EME Lab Shield TutorialBabinet s Principle: The Potential difference. Length is the Lab Shield Tutorial193 D Simulation Results: (Scott Piper)EME Lab Shield TutorialSimple Radiation Pattern20 EME Lab Shield TutorialSimple Slot Graph21 EME Lab Shield TutorialBase Line Shield22 EME Lab Shield TutorialReal Shield with Slot23 EME Lab Shield TutorialSlot Broken in Two24 EME Lab Shield TutorialSlot Broken in Four25 EME Lab Shield TutorialSummary Graph26 EME Lab Shield TutorialCST Microwave Studio Simulation of a RCD Shield with various lifted terminations:27 EME Lab Shield TutorialEME Lab Shield Tutorial28 Shield MaterialsKey Issues Determining Shield Performance: Shield Materials29 Conductivity ( (The measure of the ability of a material to conduct an electric current.))

6 Permeability ( The measure of the ability of a material to support the formation of a magnetic fieldwithin itself.)EME Lab Shield TutorialRequired Material Size for Equivalent ConductivityThese squares are different metals sized for constant conductivity: =l/(RA)Where:R is the electrical resistance of a uniform specimenlis the lengthA is the area 30 EME Lab Shield TutorialKey Issues Determining Shield Performance: Shield the Shield and (important for low frequency magnetic field applications). (which always impact negatively Shielding Effectiveness). Fieldor Far FieldEmissions (where is the source of emissions?)

7 EME Lab Shield TutorialShielding in a Nutshell:How do Shields Work?32 Reflectionat the boundary surfaces (Low Frequencies)Absorptionas fields attempt to transverse the Shield (High Frequencies)Magnetic Field Shunting (Very Low Frequencies)EME Lab Shield Tutorial33 How Do Shields Work? Reflection and Absorption in a Near or Far FieldEME Lab Shield TutorialNotes on Near and Far Field EmissionsEME Lab Shield Tutorial34 99% of the Emissions Under the Shield will be Near Field Magnetic (Switched Mode Power Supplies) Electric (DDR RAM, Micro) 85 90% of the Emissions Outside of the Shield will be Far Field The External Near Field Exception: Handheld Antenna Testing:Basic Shield Effectiveness Formulas:35 SEdB= 20 log10(Et/Ei) (Electric Field)SEdB= 20 log10(Ht/Hi) (Magnetic Field)Where.

8 SEdBis the Shielding EffectivenessEiis the Incident Electric WaveEtis the Transmitted Electric WaveHiis the Incident Magnetic WaveHtis the Transmitted Magnetic WaveEME Lab Shield tutorials . A. Schelkunoff Shield Effectiveness Equation:36 SEdB= RdB+ AdB+ MdBWhere:RdBis Reflected losses at the outer and inner Shield surfacesAdBis the Absorption loss through the materialMdBis the additional losses of Multiple reflections and transmissions within the Shield **MdBcan be disregarded for Shield thicknesses that are much greater than a skin Lab Shield TutorialShielding Effectiveness Equations Changeif the emissions are Far Field or Near Field The boundary between the Far and Near Field is approximately o/2.

9 Far and Near Field sources have differing source characteristics and E/H Lab Shield TutorialReflection Loss (RdB):EME Lab Shield Tutorial38 Occurs at a Boundary Where the is a difference in the Conductivity ( and Permeability ( ) of Two Materials (Air and Shield ) The Greater the Difference, the Greater the Reflection Loss Low Frequency Dominant(Switch Mode Power Supplies)Reflection Loss (RdB) General Formula for Far Fields:39 RdB=168 + 10 log10( r/ rf)Where: r= Conductivity relative to Copper r= Relative permeability relative to free spacef = FrequencyNote, Reflection loss is greatest for:Low Frequency (f)High Conductivity ( r)Low Permeability ( r)The larger the RdBthe better the Lab Shield TutorialFar Field Reflection Loss (RdB) Example:40 Material r rRdB@1kHzRdB@10 MHzCopper 138 dB98 dBNickel 126 dB86 dBSteel1000 98 dB58 dBRdB=168 + 10 log10( r/ rf)EME Lab Shield TutorialAbsorption Loss (AdB).)

10 41 Absorption Loss is the exponential decay of energy due to ohmic and heating of the material which occurs when an electromagnetic wave passes through a Frequency Dominant(Video Data, DDR, Micro Data Communications)EME Lab Shield TutorialA little aside, Skin Depth ( )42To understand Absorption Losses, there is a need to understand the term Skin is Skin Depth? The distance required for the wave to be attenuated to 1/e or 37% of its original value is defined as the Skin Depth ( ) which is: = (2/ ) (meters) Or: = (f r r) (inches)Remember this term: (Skin Depth) it is Lab Shield TutorialBack to Absorption Loss (AdB):43A = et/ Or in dB:AdB= 20 log10et/ Where:t = thickness Skin DepthNote, Absorption Loss is greatest for.