Date: Mon May 15 2000 - 19:59:09 PDT
In a message dated 5/15/00 1:55:52 PM Pacific Daylight Time,
<< How does the "chassis" help control EMI? Isn't the Faraday cage the
This question was in regard to the "chassis ground rings" I recommend around
the periphery of high-speed printed circuit boards (PCBs).
Through an accumulation of design techniques that have been implemented as
patches to resolve a particular problem. I have strived to implement
multiple combinations of "patches" in easily manufacturable structures to
optimize cost versus performance. Some patches may have to stand alone, but
the PCB rings do not. I have often referred to the chassis rings as forming
a Faraday cage around the PCB edges; however, this is an exaggeration. The
end result is similar, but is achieved in a roundabout way.
The via-interconnected (stagger via spacings to avoid a resonant ladder
structure at high harmonics) rings provide a relatively low inductance
conductive structure around the PCB edges. As a minimum, this shield
structure must be grounded to the signal reference at the I/O connectors.
The rings provide a termination point for errant E-fields emitted from not
only the PCB edges, but also other surface conductors on the PCB interior.
The currents sourced by these fields travel along the rings to the closest
signal reference connection and then back to the original source within the
PCB. This technique serves to locally contain the PCB-generated fields (as
observed by other subassemblies, cables, or other conducting structures in
the total assembly).
Further benefits are also realized when the rings are tied to the conducting
chassis (if one exists) surrounding the entire PCB subassembly. If a
conductive enclosure exists, It serves as a more perfect Faraday cage
surrounding all fields emanating from all subassemblies within the enclosure.
I recommend low inductance connections of the chassis rings to any enclosure
to establish an extension of the Faraday cage of the enclosure to the
immediate vicinity of the PCB. This technique helps reduce field coupling
between subassemblies. These connections are most commonly at the I/O ports
to help create "quiet" ground points that help reflect both external and
internal common-mode waves. Additional connections at PCB mounting points
are also beneficial.
Many (particularly older) subassemblies use low frequency circuits and
packaging techniques. Unfortunately, they are susceptible to picking up
higher frequency radiations from adjacent PCBs and can then conduct that RF
energy to the outside world via their low-frequency I/O ports. The "chassis
ground rings" on high-speed PCBs have saved many of my clients from having to
redesign older, low-cost PCBs (that are contained within the same assembly)
to meet EMC requirements.
One could go on for a chapter or two on this subject, but enough is enough
Michael L. Conn
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