EMI Analyst™

EMI ANALYST™ Software SuiteI n t u i ti v e A c c u r a t e E ff e c ti v e

How to extract shielded cableproperties from data sheetsA radiated susceptibility example.Applicable to all four types of electromagnetic interference analysis.

EMI Analyst™

The task is to characterize cables for EMC• Often we have to determine if

a certain cable is adequate for the environment• How to input cable

characteristics into EMI Analyst to perform RE/RS type tests? • Published cable spec is shown

at right.• It incompletely defines cable

characteristics

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What do we need to know?

Conductor properties Transfer Impedance properties

Several physical properties are needed before cable electrical characteristics can be calculated.

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Known cable propertiesWhat we know

• Coaxial cable• 50 ± 2 characteristic impedance• 90 pF/m• #29 AWG center conductor• Nickel-plated copper covered steel• Extruded TFE dielectric• 90% optical coverage shield• Nickel-plated wire shield

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Conductor form properties Cable configuration-

Shielded wire over ground plane

Wire material - Nickel Wire gauge - #29 AWG Ctr-Ctr Offset - 0.0 mm, (coax) Insulation - Extruded TFE (Teflon) Height - 5 cm Length - 2 m Shield Diameter - Derived Shield Thickness - Estimated Shield Material - Nickel

Click links to see rationale for each property selection

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Transfer Impedance form properties Braided (woven wire) Strand material - Nickel 1 mm shield diameter - Derived #38 AWG - Assumed 32 strands max. - Derived 90% optical coverage

4th option on Transfer Z form is best fit for known properties

• Carriers x Ends < Optical Coverage x Shield Dia. / Strand Dia.

Click links to see rationale for each property selection

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1 mΩ

50 Ω

1 mΩ 1 mΩ 1 mΩ

50 Ω 5 cm

2 m

50 Ω 50 Ω

Analysis model for evaluating shieldStrategy

• Expose cable to 1 V/m, 2 MHz to 18 GHz• See how much

current flows on center conductor• For perfect shield, no

current flows on center conductor

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EMI Analyst Model

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Frequency Range2 MHz – 10 GHz

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Limit1 V/m from 2 MHz to 10 GHz

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Field DirectionBroadside, Vertical Polarization

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Left-Hand End Circuit50 Termination, Chassis Referenced

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Cable ConductorsAs Defined in Previous Slides

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Shield Transfer ImpedanceAs Defined in Previous Slides

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Shield Terminations360-Degree (Coax), 1 m

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Right-Hand End Circuit50 Termination, Chassis Referenced

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Nickel Braid Shield, Induced Current

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Copper Braid Shield, Induced Current

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Nickel Solid Shield, Current on Wire 1

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Property SlidesThe slides below are linked to slides 5 and 6.

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Conductor material• Center conductor is nickel-plated copper-covered steel, shield is nickel-plated

wire• RS frequency range is 2 MHz to 18 GHz

• At 2 MHz, skin depth is 3.8 m• Minimum plating thickness is 1.25 m for nickel

• Over most of the RS frequency range, current flows primarily in nickel plating, so use nickel for conductor material

http://chemandy.com/calculators/skin-effect-calculator.htm

http://www.fiskalloy.com/products/electroplating/

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Standard nickel plated conductors have a minimum plating thickness of 50 micro-inches (1.25 microns) as specified in ASTM B 355.

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Dielectric properties• Extruded TFE• DuPont TFE (Teflon)

• r = 2.1• 1 mm dia. shield -> 50 char. Z• Very small coax

• Halocarbon TFE• data sheet at right• r = 26.14• 21 mm dia. shield -> 50 char. Z• Seems too large for #29 center

• Assuming DuPont Teflon

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Test setup – MIL-STD-461F (assumed)

• Cable configurationShielded wire over ground plane

• Height• 5 cm per MIL-STD-461

• Length• 2 m per MIL-STD-461

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Shield diameter• Derived from known properties

• #29 AWG center conductor• Teflon dielectric• 50 characteristic impedance• 90 pF/m

• Shield Diameter = 1mm• Found using TAEM*

• Higher characteristic impedance at low frequencies is due to resistance of nickel conductors,

*TAEM = trial-and-error-method. Try shield diameter, then calculate Z0. Repeat until correct shield diameter found.

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EMI Analyst™

Shield thickness• For braided wire shield, shield

thickness = strand diameter• Shield diameter is 1 mm• Estimate shield is #38 AWG strands• 100.72 m dia. strands• 32 woven strands, max.

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