ECE4122 Chap.10 Shielding and Filtering
Shielding and Filtering
A shield reduces coupling by reflection, absorption and control of current paths. Shielding effectiveness is a decibel ratio between fields or powers measured under defined conditions. Material conductivity and permeability influence penetration, but enclosure seams, apertures and cable entries often dominate practical performance.
A conductive box with an untreated cable penetration can route disturbance directly across its boundary.
Skin depth decreases with frequency, conductivity and permeability. A thickness of several skin depths provides strong absorption for a plane-wave model, but near-field magnetic shielding at low frequency may require high permeability, increased separation or source-loop reduction.
Aperture effectiveness depends on its largest dimension, orientation and repetition, not just total open area.
Filters provide frequency-selective impedance. Insertion loss compares transmission with and without the device in a specified source-load system.
A nominal component value is insufficient at high frequency because lead inductance, capacitor equivalent series inductance, winding capacitance and layout create resonances. The installed filter must be evaluated as a network with realistic terminations.
Placement determines whether unwanted current crosses a protected boundary before being diverted.
Feedthrough capacitors and connector filtering work best when bonded at the entry plane, keeping dirty and clean current paths separated. A sound EMC design coordinates shielding, bonding and filtering so that the enclosure surface current has a continuous route and the filtered conductor does not recouple downstream.
What this chapter covers
- 01
Shielding-effectiveness definition
- 02
Skin depth and absorption
- 03
Seams, apertures and penetrations
- 04
Insertion loss and parasitics
- 05
Boundary filtering and installation
Worked example · free
Convert attenuation to a field ratio
- +1For a field amplitude ratio, SE = 20 log10(Eincident/Etransmitted).
- +1Rearrange to Etransmitted/Eincident = 10^(-SE/20).
- +1Substitute SE = 46 dB.
- +1The ratio is 10^(-2.3), approximately 0.00501.
- +1Thus about 0.50% of the incident field amplitude remains under the stated test definition.
Key terms
- Shielding effectiveness
- A logarithmic reduction in a specified field or coupled quantity.
- Skin depth
- The conductor depth over which field amplitude falls by a factor of e.
- Aperture coupling
- Field transmission through a seam, slot or opening.
- Insertion loss
- The transmission reduction after a device is inserted into a test system.
- Feedthrough capacitor
- A boundary-mounted capacitor that diverts high-frequency current at entry.
- Self resonance
- The frequency where component parasitics change its intended behaviour.
Shielding and Filtering FAQ
Why can a thick enclosure still leak?
Seams, slots, displays and cables can dominate coupling even when the bulk wall has excellent absorption. Leakage is then controlled by current continuity and aperture geometry rather than transmission through the bulk material.
Is filter insertion loss universal?
No. It depends on frequency, source and load impedances, topology, parasitics and installation geometry. It must be quoted with the measurement impedances, reference planes and installation configuration that produced it.
Where should an entry filter be mounted?
Mount it at the shield boundary with a low-impedance bond so disturbance current is diverted before entering the protected volume. This prevents the unfiltered lead from carrying disturbance into the enclosure and recoupling around the network.
Exam move
Treat each remedy as a current-path design. For shields, identify surface-current continuity and every penetration. For filters, draw source, load and parasitics, then explain where unwanted current goes above and below the transition frequency. Organise shielding questions around the actual current and field path. Identify whether the source is predominantly electric, magnetic or plane-wave-like at the shield location.
Mark the enclosure surface-current route, every seam and aperture, and each cable crossing. Then decide whether material reflection, absorption, magnetic permeability, increased separation or aperture control addresses the dominant mechanism. A bulk-material attenuation calculation is not a complete enclosure result when current can bypass the wall through a connector or long slot.
For numerical work, declare whether a decibel value compares field amplitudes or powers. Convert shielding effectiveness or insertion loss back to a ratio to test its scale. Calculate skin depth with consistent conductivity, permeability and angular frequency, then compare shield thickness with several skin depths only within the assumptions of that model.
At low frequency near a magnetic source, explain why high conductivity alone may not deliver the predicted plane-wave result. Treat filters as installed two-port networks. Draw source impedance, filter topology, load impedance and the high-frequency return. Calculate an ideal corner only as the first step.
Add capacitor series inductance, inductor winding capacitance, resistance and layout coupling to predict self-resonance and degraded stop-band behaviour. Use impedance-versus-frequency or network data at the interference band. A nominal component value without its parasitic regime is not enough to justify attenuation. Placement should follow the protected boundary.
Bond cable shields and shunt elements at entry so unwanted current is returned on the dirty side before the conductor can radiate inside. Keep filtered and unfiltered wiring physically separated to prevent recoupling. Verify with the same source and load conditions, measurement reference plane and detector bandwidth before and after the change.
If conducted current improves but radiated emissions do not, investigate common-mode cable current or aperture leakage rather than adding more filter order blindly. Close each design by naming what must pass, what must stop, where the rejected energy flows and which measurement demonstrates that outcome.
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