Re: [SI-LIST] : Trace Impedance Selection

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From: Neven Pischl (npischl@cisco.com)
Date: Sat May 13 2000 - 09:10:47 PDT


----- Original Message -----
From: Vinu Arumugham <vinu@cisco.com>
To: <si-list@silab.eng.sun.com>
Sent: Friday, May 12, 2000 12:23 PM
Subject: Re: [SI-LIST] : Trace Impedance Selection

> Imagine a wide trace high above a plane and a thin trace close to the
plane both with the same Zo. The wide trace can be viewed as several large
loops each carrying a small current while the thin trace is one small loop
carrying a large current. Would you expect to see a difference in the
radiation?
>
> Thanks,
> Vinu

Having all other factors the same (including the total current in each
trace), I would expect to see the difference. I would expect the trace that
is higher above the plane to radiate more if the traces are either
electrically short, or if they are electrically very long. At the lengths of
the traces that are just around the first couples of resonances, I would
expect that is may be possible that the lower trace radiates more at some
frequencies where it becomes tuned while the other is not. I also guess that
how pronounced this effect may be would be a function of the difference in
height. I am not sure how strong this effect might be, but that is what I
guess, speculate ..... Haven't done it.

The above model with the wires is just a very rough approximation, which may
be okay for the analysis of radiation. But beware of the current
distribution which should be taken into account to obtain correct Zo and
close-proximity effects, such as crosstalk, in which it wouldn't work.

I don't know if you already have it, but I can recommend a good reference
(at least I like it a lot):
"Fields and Waves in Communication Electronics" by Ramo, Whinnery, and Van
Duzer, John Wiley & Sons, ISBN 0-471-58551-3.

Neven

>
> Neven Pischl wrote:
>
> > To a certain extent these two things can produce same effects, but they
are not quire the same. E.g. if you want to reduce emission from a trace by
getting it closer to its reference plane (return current), the
characteristic impedance will go down if you keep the same trace width.
However, you can keep the Zo constant by shrinking the trace width, to the
limits allowable by manufacturing, which is in most "standard" cases 4 mils.
Hence, you can keep the same impedance and still reduce radiation from the
trace. By the same token, even increasing the Zo can result in lower
emission if the geometry of such and antenna becomes an inefficient
radiator. Try to visualize a very thin line, very close to a plane. It can
result in a high Zo, but due to the close proximity to the plane it will not
radiate (as much as it would if you lift it higher above the plane).
> >
> > As I wrote in my previous mail, in the part that got clipped from your
response below, one has to look at the overall picture, and the impedance
itself does not tell anything about the radiation.
> >
> > I see some similarity with the ways we can connect capacitors to a
board. This reminds me of often seen practice to connect decoupling
capacitors with very wide traces. No matter how wide the traces are, if the
loop that current passes through is large, it will not work well. That is
because the inductance is primarily determined by the loop area, not by the
width of conductors. The width comes in play only as a second-order effect.
Similarly, the radiation from a trace (which is, among the others, a
function of the area below the trace) will be in the first-place determined
by the more "general" geometry, such as the height and length. Zo is also a
function of it, but there is no direct correlation between two of these.
> >
> > Interestingly, I have never received my original post, the part of which
is below. Thus I didn't even know that it was actually posted on the list,
until I got this response from Vinu. Do the other members of the list
experience the same effects occasionally?
> >
> > Neven
> >
> > At 10:49 AM 5/11/00 -0700, you wrote:
> > >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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