RE: [SI-LIST] : Trace Impedance Selection

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From: sweir (weirsp@atdial.net)
Date: Mon May 08 2000 - 11:53:29 PDT


Mary,

Gross EMI is going to come from microstrips. However, striplines are a
concern as well. I think Michael has pointed out the common ways that
striplines can radiate, mostly due to the "impurities" of the real
world: vias, surface break-outs, proximity to the board edge, component
lead structures, etc.

Striplines have crosstalk considerations. There are various techniques for
making striplines more coax-like, including via fences that reduce some of
the problems due to the impurities Michael has described.

Regards,

t 09:49 AM 5/8/00 -0500, you wrote:
>Steve,
>
>Thanks for responding. I appreciate your insight. My background
>is more theoretical and I wjould like to learn more about how SI
>engineers view things.
>
> >As much as I agree with that aspect, the notion I believe Vinu was
> >attempting to address, ( and I believe of practical value ) is that the
> >flux density surrounding T-line on a PWB as typically a microstrip or
> >stripline goes up as the impedance goes down and vice-versa.. For those
> >constructions a significant amount of the energy transmitted down the line
> >does so in the dielectric surrounding the signal and return etch. That
> >flux is free to intercept any antenna we place in the field, whether it is
> >a chassis slot, or a susceptor trace.
>
>I had not considered relating the characteristic impedance to a
>concentration of flux. This seems reasonable for microstrip geometries.
>I will give this some more thought.
>
>You also mentioned striplines. Are you suggesting that the radiation or
>coupling from a stripline is proportional to the characteristic impedance?
>This seems anti-intuitive to me.
>
>Mary
>
>
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