Route the signals on 45 degree angles
and your problem is solved!
(just kidding)

Jack



On 3/15/07, Post, Devon <[log in to unmask]> wrote:
>
> Jack,
>
> Thanks for your reply.  The purpose of the board would be for 40+ Gbps
> signals.  Unfortunately at these speeds anything with glass weave starts
> to affect the signals.  The purpose for teflons is the loss tangents are
> very very low and, depending on the material, there is little to no
> glass weave.
>
> (for Bill Brooks)
> The last part of my statement about little to no glass weave is the
> important part here that differentiates what we talked about a couple of
> weeks ago.
>
>
> For those that would like to know more.
>
> The glass weave in laminates start to affect the signals at higher
> frequencies say above 10 GHz.  The reason has to do with the signal
> traces and uniformity about the weave.  The weave of the glass ebbs and
> flows like a wave, meaning there are peaks and valleys.  If one of the
> signals of a differential pair were to be manufactured such that it
> resides over more valleys than it does peaks, and the other signal was
> over more peaks than valleys, the result would be that electrically at
> high frequencies the signals would not be aligned (this is an
> over-simplification of what is really going on).  Even if the cad tool
> routed them to within a mil, or 25um, of each other, they could still be
> skewed significantly from a phase angle perspective.  Why is this true?
> The instantanious dielectric constant that the signals see would be
> different.  This causes the propagation velocity of the signal to be
> different for the two traces.  At low frequencies we assume that the
> dielectric is uniform and therefore the dielectric constant is uniform,
> at high frequencies this is not as true.  The end result is that the
> differential pair will have greater mode conversion, and there will be
> additional attenuation of the signal as it propagates through the board.
> This issue will need to be addressed during the design phase of a board
> as signal frequencies continue to increase in speed.
>
> Devon
>
> ========================================================================
> Devon J. Post - C.I.D.                           Mayo Foundation
> <mailto:[log in to unmask]>                     200 First Street SW
> Voice:  (507) 538-5479                           Sn 2-134C
> Fax:    (507) 284-9171                           Rochester, MN 55905
> Internet:  http://www.mayo.edu/sppdg/sppdg_home_page.html
> ========================================================================
>
>

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