[SI-LIST] Re: Traces don't cause EMI - really?
- From: MikonCons@xxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 22 Oct 2003 18:56:57 EDT
In a message dated 10/22/2003 2:43:36 PM Pacific Standard Time,
alexh1@xxxxxxxxxxxxx writes:
I routinely see requirements for low cost office equipment that the case be
non-metalized plastic. And this is with SDRAM memory interfaces running at 100+
Mhz. It's not necessarily easy, and it may be impossible for a multi-vendor,
multi card, multi cable environment like a PC. If one is designing a card for
a PC, he/she may not be as attentive to EMI issues knowing it will operate in
a metal enclosure.
********
Your "multi-vendor, multi-card" comment is right on target. I have redesigned
plastic-encased, fully self-contained units to pass FCC Class B by building
the Faraday "enclosure" right on the outer layers of the PCB. However, the
common plug-in cards of normal PCs depend on using a shielded enclosure around
the
entire assembly and generally have not been designed for stand-alone
compliance.
My military product design experience strongly supports designing daughter
cards that are inherently shielded to avoid coupling between adjacent cards. Of
course, military requirements are more demanding relative to consumer
applicatiions.
NOTE: The loop antenna formed by a single-ended trace over a ground plane
radiates normal to the PCB. Likewise, the single-ended traces on an adjacent
card
RECEIVE best (and are therefore most susceptible) for normally impinging
fields. The result is that microstrip-based designs DO radiate sufficiently to
cause EMI problems on adjacent microstrip-based cards. That's another reason I
recommend stripline for ALL fast-edge signal traces. These days, that criteria
covers most signals.
Mike
Michael L. Conn
Owner/Principal Consultant
Mikon Consulting
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