Re: [SI-LIST] : x-talk saturation

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From: opamps cybernetics (opamps@hotmail.com)
Date: Tue Aug 22 2000 - 08:25:24 PDT


Jim,

1)
You wrote:
>The saturation occurs when the modes separate, before
>that you will see a mixture of both modes.

"Saturation occurs when modes separate", Why?

2) Are you saying there is no saturation on a stripline?

Opamps

>From: jvercamm@roam.agfa.be (Jan Vercammen)
>Reply-To: jvercamm@roam.agfa.be (Jan Vercammen)
>To: si-list@silab.eng.sun.com
>Subject: Re: [SI-LIST] : x-talk saturation
>Date: Mon, 21 Aug 2000 14:47:09 +0200
>
>opamp,
>
>the effect you are describing is due to differences in mode velocities.
>
>Assuming you have a lossless muliconductor system with n+1 conductors,
>there
>will be, in general, n linear independent propagation modes. In an
>homogeneous
>medium (e.g. stripline or infinite large medium) all modes travel at the
>same
>speed, however, in an inhomogeneous medium (microstrip, embedded
>microstrip)
>the modes have different propagation speeds.
>
>When a generator (or generators) excite(s) the multiconductor lines n modes
>will be excited, each propagating at a different velocity. For a finite
>rise time
>the modes will separate after covering some distance.
>
>For example: consider a symmetric 2 conductor line + reference plane
>(return). This
>2+1 conductor system has two propagating modes. In case of a symmetric
>system the
>two modes are equivalent to the (better known) odd and even mode, which
>makes this
>easier to discuss. For non-symmetric systems the situation is slightly more
>complex,
>but quite similar to the symmetric system.
>
>Depending on the (inhomogeneous) medium the odd mode velocity will be
>faster slower
>than the even mode velocity. For microstrips (PCB technology) the
>difference is of the order
>of several ps/cm. For a rise time of 200ps and a mode velocity difference
>of 5ps/cm
>the modes will separate at a distance of 200/5=40cm. Before this distance
>the
>propagating field and the associated voltages and currents will be a
>superposition of the
>odd and even mode, after this distance the modes become more and more
>separated as
>the covered distance grows.
>
>The saturation occurs when the modes separate, before that you will see a
>mixture of
>both modes.
>
>You can generalize the above discussion to an n+1 multiconductor system. In
>general you
>will see n levels, the first level is due to mode k, the second level due
>to modes
>k and l, ..., the final level due to all modes. Note that the level of the
>agressor
>and culprit lines can take on various levels and it could swing positve or
>negative before
>settling in the last final settings.
>In a homogeneous system you will only see one level, that is, the
>superposition of all modes.
>
>In most practical situations the details of the modes are very hard to
>observe, because (1)
>differences in mode velocities are small (or coupling lengths are short)
>and (2) there is
>attenuation.
>
>If you want to go deeper into this matter then you should consult the
>following
>reference: Analysis of multiconductor lines, author C.R. Paul (there are
>very likely other
>good references).
>
>regrads,
>
>Jan Vercammen
>Agfa-Gevaert
>EMC Engineering
>
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