[SI-LIST] Re: EMI fixed by flooding?
- From: "Vishram Pandit" <vishrampandit@xxxxxxxxxxx>
- To: BDempsey@xxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 30 Jan 2003 23:22:37 +0000
Bill,
My observations are as follows:
1] The forward current path and return current path needs to be closely
coupled.
2] If the frame ground is just tied to one point, then it may offer higher
impedance. As you suggested, the cables may have lower impedance for
emissions.
3] It will be helpful, if it is tied to multiple points. That way, you will
avoid loops.
4] It is recommended to alleviate the EMI at the source level. The power and
ground impedance at the sources need to be optimized.
5] EMI is a combination of the radiation from the traces and from the edges.
The floded outer plane may solve the problem of trace radiation from the
surface, but it will not address the edge radiation issue.
Thanks,
Vishram
>From: Bill Dempsey >Reply-To: BDempsey@xxxxxxxxxxxx >To: "Si-List (E-mail)"
>Subject: [SI-LIST] EMI fixed by flooding? >Date: Thu, 30 Jan 2003 16:19:22
-0600 >>>Last summer there was a good thread going around about ground
>floods and their effect on EMI. I am interested in >finding out the
opinionsof those who care regarding the >following problem. >>First some
sarcasm. We all know that good design is verboten when >the band-aid box is
full. Given this I'm been given a severly failing >design, a box of
band-aids, and a set of rules I'm supposed to implement. >(By severely
failing I mean the product did not pass FCC part A emissions >because of
large emissions at the system clock frequency.) >>Back to the rules. The
rulethat's got me uncomfortable is I've been told to >flood >the outer
layersof a backplane (the culprit) with Frame gnd. This will fix >the EMI
failure. Ok... does it? >>Some background: >>The box that this backplane is
going in is metal on several sides but >has openings for cards in the front
and cables in the back. >>The box is tied to Frame (i.e. Earth). >>Digital
GND (just a name to imply not Frame GND) is used for signal >return on
*some*layers of the backplane. Low voltage power is used >for signal return
on other layers. Not all signals that transit the >backplane >use the same
common reference. Hmmm... >>Currently the outer layers are microstrip
(soldermask coated). >If I flood the outer surfaces of the backplane with
FGND and I have >a dielectric thickness of around 6 mils to DGND haven't I
just >introduced a great AC path to FGND that would not have existed if the
>box metal was allowed to float? >>And by doing so doesn't this help
increasechances of emissions? And if a >"farday cage" >is open on two ends
does this concept help? >It seems to me that the cause of failure has to do
more with the fact that >they >have a gazillion (power of ten higher than
yotta) cables going out of this >chassis and >the return energy for the
clockhas found a lower impedance path across the >cables and >therefore has
created a loop which is radiating. >>Thanks, >Bill
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