The myth: The representation of shield performance (in dB) as applied to products will be identical for EM fields developed internally to the product, compared to fields externally impinging on the shield.
The reality: The performance of the shield will probably be very different for the two field conditions, perhaps by significant performance difference.
Shielding performance essentially is a transfer function response to the propagation of electromagnetic fields that are impinging upon the shield. For shielding, the intent of the transfer function involves optimizing the mismatch of impedance. This mismatch may be both from the electromagnetic wave impedance compared to the shield impedance and, within the shield material itself, from the boundary of the shield surfaces compared to the “core” impedance of the material (assuming that the material has multiple skin depths of thickness).
The first effect of any shield in the process of transfer functions is to mismatch the impedance of the impinging electromagnetic wave at the boundary surface of the shield. As with any impedance mismatch in a transmission line, the mismatch causes a reflection loss. The greater the ratio of impedance mismatch, the greater the reflection loss from the shield will be. When shields are thick enough to present multiple skin depths at the frequencies of interest, and assuming that the shield metal is a “sandwich” of highly conductively plated surfaces (such as electro-tin) applied to a different material (such as cold rolled steel), an additional transfer function of loss is noted. This loss is the second effect of shielding and is observed as an additional inter-boundary impedance mismatch within the material itself. These mismatches set up greater shielding performance by promoting losses within the material.
As a consequence of these processes, it can be conceptualized that the ratio of the various mismatches in the transfer functions holds the key to shielding performance.
Consider that to contain/capture within a product electromagnetic waves that are sourced from the circuits and circuit boards of the product, the dimensional proximity from the sources to the shield surfaces will be relatively small (often less than 1 cm). As a process of the transfer functions and as described in our tutorial program, EMCT (Module 3), the wave impedance from the internal sources to the shield will probably be found in the region of approximately 10 to 50 ohms, (near-field, magnetic-dominant mode). Should the impedance of the shield material be approximately 5 ohms, the anticipated first effect of the reflection loss (10 / 5 to 50 / 5 = 2:1 to 10:1) will be in the range of 6 to 20 dB.
Contrast this performance against what would occur if the electromagnetic wave impedance was sourced in the far-field, tens of meters away from the product shields. Under that condition, the electromagnetic wave impedance will probably be that of the impedance of space, approximately 377 ohms. The first effect performance of the shield will now be based on an impedance reflection ratio of about 75:1 (377 / 5) or 38 dB, even without considering other transfer function losses that may be evident within the material properties.
These significant differences in shield material performance also explain why shields may exhibit very different performance values when used and measured on a product, compared to the values represented by a shielding manufacturer. This is because the product may present to the shielding surfaces a very different electromagnetic wave impedance characteristic compared to that used for evaluation of the materials themselves for “catalogue” purposes.
|W. Michael King
is a systems design advisor who has been active in the development of over 1,000 system-product designs in a 50 year career. He serves an international client base as an independent design advisor. Many terms used for PC Board Layout, such as the “3-W Rule”, the “V-plane Undercut Rule”, and “ground stitching nulls”, were all originated by himself. His full biography may be seen through his web site: www.SystemsEMC.com. Significantly, he is the author of EMCT: High Speed Design Tutorial (ISBN 0-7381-3340-X) which is the source of some of the graphics used in this presentation. EMCT is available through Elliott Laboratories/NTS, co-branded with the IEEE Standards Information Network.