Re: [SI-LIST] : Trace Impedance Selection

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From: Vinu Arumugham (vinu@cisco.com)
Date: Fri May 12 2000 - 12:23:42 PDT


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

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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