RE: [SI-LIST] : Trace Impedance Selection

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From: Heyfitch, Vadim (vadim.heyfitch@intel.com)
Date: Fri May 19 2000 - 16:19:23 PDT


        I would like to add my 2 cents to this discussion.
The original issue was as the following: having to preserve constant
impedance, is it better to have a thin trace closer to the ground plane, or
a thicker (scaled up) trace farther above the plane. "Better" was defined as
having least radiation from this microstrip topology.

Although many opposing ideas have been voiced so far, all seem to have
missed the key point. The TEM currents BY DEFINITION are equal in magnitude
and opposite in direction on two conductors that make up a transmission line
(in this case a trace and a plane.) That is true at ANY crossection along
the line. The radiated fields from these currents completely cancel out each
other in far field. When I say "completely" I mean that a combined field
(from the two currents) falls off as some power of (h/d) as d->oo (here d -
distance from the microstrip to the point in space where the field is
observed/measured; h - height of the trace over the plane). Of the top of my
head, I cannot speculate what power this is. But, in any case, whether it's
3 or 4 or 6, the combined far field asymptotically vanishes. Therefore, the
two cases in hand are different in that the far field falls off faster for
the thinner trace sitting lower above the plane. As to the issue of
impedance effect on the far field.... Naturally, the far field E(d>>h)~
I*(h/d)^n is proportionate to the current I, or some power of I. (Here,
again, I emphasize that I is the TEM current, or DIFFERENTIAL MODE current).
Therefore, a field from a lower impedance line (i.e., higher current) would
fall off slower.

Antenna current, on the other hand, is the REAL cause of measurable
radiation. This current is BY DEFINITION a common mode current. It is
usually much smaller than the TEM currents. It's caused by any deviation
from ideal transmission line symmetry in actual design. Therefore, to fight
emissions one should concentrate on making transmission lines as ideal as
possible to eliminate/prevent any causes for common mode (or antenna - same)
currents.
Due diligence in this venue will bring a much larger pay off than making
high-Zo-narrow-and-low-sitting traces.

If interested further, this issue is lucidly explained in sec 1.5.2 of
"Analysis of Multiconductor Transmission Lines" by Clayton R. Paul.

-Vadim
SWCD, Intel

> -----Original Message-----
> From: Vinu Arumugham [mailto:vinu@cisco.com]
> Sent: Thursday, May 11, 2000 10:50 AM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Trace Impedance Selection
>
>
> Neven Pischl wrote:
>
> > What really matters in reducing EMI is to build very
> inefficient antennas,
> > by keeping them (the traces) close to the reference planes
> - thus reducing
> > the loop area and crosstalk as well, by assuring that the
> return currents
> > flow adjacent to the traces, by keeping them short ....
>
> "Keeping traces close to the ref. planes", "reducing loop
> area", are these not
> the same as saying lowering the characteristic impedance of
> the line reduces
> EMI?
>
> Thanks,
> Vinu
>
>
>
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