RE: [SI-LIST] : Trace Impedance Selection

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From: Mary (mary@advocate.net)
Date: Mon May 08 2000 - 07:25:20 PDT


Mike,

Thanks for your very thorough response. Your comments did not seem harsh
at all. In fact, I do not disagree with most of what you said. You said:

>If your last sentence quoted above were true, a lot of high-priced
>software that purports to predict radiated emissions from circuit boards is
>taking the industry on a scam ride.

I am not familiar with the inner workings of software that purports to
predict the radiated emissions from circuit boards, but my initial reaction
is that software that predicts the radiation from microstrip patches is
highly complex and requires a great deal of computer resources. Microstrip
patch antennas are much simpler than printed circuit boards, so I am
guessing that software that purports to predict radiation from printed
circuit boards is vastly oversimplifying things.

>The far-field radiation from a one inch surface trace on a printed circuit
>board is predicted by EMCAD1 software (produced by CKC Labs) to be 4.75 dB
>higher for a 119 ohm line relative to a 69 ohm line; i.e., the higher
>radiation is proportional to the log of the ratio of the two impedances.
The
>actual equation that computes the radiation for a given line uses the log
of
>the ratio of the trace characteristic impedance to the impedance of free
>space (120*Pi=377 ohms). The closer (higher) the line characteristic
>impedance is to that of free space, the higher the radiation. The lines
>noted were 5 mils wide and use an Er of 4.5. The 69-ohm line used a 5-mil
>dielectric thickness, and the 119-ohm line used 20 mils.

So apparently this software predicts infinite radiation from a 377-ohm line?

The radiation from a microstrip trace is not proportional to the log of
the characteristic impedance. In fact, for a narrow trace it is
approximately proportional to the height of the trace above the plane
while the characteristic impedance is proportional to the log of the
height. The radiation expressed in dB would be approximately proportional
to the characteristic impedance for a fixed trace width. Perhaps this
is what the software is assuming. If so, the expression used by the
software is a convenient approximation that is only valid for a fairly
specific geometry.

I do not want to suggest that anybody's software is incorrect. My point is
that the characteristic impedance of a transmission line does not convey
information about how much the line radiates.

Thanks again for responding to my post. I am enjoying the discussion (and
learning a lot).

Mary

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