[SI-LIST] Re: Traces don't cause EMI - really?
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 22 Oct 2003 07:23:48 -0700
Please clarify any points I misrepresent below. They are a compilation =
of the most salient points I know of concerning this thread.
1) Microstrip traces can radiate EMI, as indicated by the Dockey paper =
and others. =20
2) I don't believe a case of "well behaved" microstrip traces generating =
enough EMI to fail FCC standards has been demonstrated - all failing =
instances seem to be accompanied with the caveat of including a design =
error, or unreasonable geometries (.062" above the ground plane).
a) This seems to agree with my experience - there are many 4-layer =
boards out there where microstrip is your only option (I would also =
point out that they don't have the luxury of closely coupling the power =
and ground planes, but that could lead to another furball).
b) If, in fact, it is possible to route high speed signals on =
microstrip and, with proper design considerations, pass FCC emissions =
standards, it would seem to be a mistake to do otherwise. I, for one, =
have had the unfortunate experience of overdesigning (to be on the safe =
side), only to find my competitor offering a product cheaper when they =
cut corners. I've also had the opposite experience - being penny-wise, =
but pound foolish. Knowing when we are being properly cautious is what =
we get paid for.
3) In the case of a not-so-well-behaved layout, which we are often =
forced into, closely-coupled differential traces offer a high level of =
"forgiveness" over single-ended, or loosely-coupled differential =
routing.
Jeff Loyer
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