RE: [SI-LIST] : GND on the outer layers

Chris Rokusek (crokusek@viewlogic.com)
Wed, 14 Jul 1999 08:35:05 -0700

Jon,

Two very dominant EMI mechansims at the PCB level occur specifically when
traces are routed on surface layers (microstrip or embedded microstrip) and
are virtually eliminated when traces are surrounded by solid planes on both
sides (stripline):

1) Differential mode radiation. Signal currents flowing in the loop area
formed by a signal trace and its return plane are not shielded by solid
planes and will radiate into the far field.

2) "Current Driven" Common Mode radiation (term used by Univ. of
Missouri-Rolla EMC Consortium). Signal currents returning through a finite
return plane induce small (common mode) noise voltages across ALL return
planes which in turn may drive attached cables. In terms of fields, think
of this as the magnetic field that wraps the entire board (esp near corners)
associated with the partial inductance of the return plane.

The expected far field radiation for these mechansims can be computed
efficiently using driver characteristics, attached cable information, and
physical layout.

Chris Rokusek
Viewlogic Systems

> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Jon Keeble
> Sent: Tuesday, July 13, 1999 7:40 PM
> To: 'si-list@silab.eng.sun.com'
> Subject: [SI-LIST] : GND on the outer layers
>
>
> I'd be interested in opinions on whether placing tracks "inside" and
> putting GND planes on the outer layers reduces raditation.
>
> It seems to me that there is little difference regarding the high
> frequency magnetic component: the return current in an outer plane runs
> parallel with the track, and the materials have little effect on the
> field.
>
> The E field is different in that the conductivity of the outer GND
> planes would confine this component completely within the PCB.
>
> Has anybody modelled this?
>
> Jon Keeble.
>
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