[SI-LIST] Re: Remove Ground underneath Differential signal is deserved or not?

Hi, Jim:

Precisely, the ground plane is an integral part of the system. When you
remove it, the even mode will not exists. In the order word, the even mode
will have a factor like exp(-alpha*z). In contrast, the odd mode is a
propagating mode and it will have a factor like exp(-jbeta*z). For the
difference, the odd mode will propagate, while the even mode will decay very
fast away from the source or the discontinuities creating the even mode. In
some sense, if you have a differential pair and you have some
discontinuities at the middle of it, the signal will first have only
differential signal coming into the discontinuties, the discontinuites will
try to create the even mode. However, the even mode can not be built up due
to the fact it is not propagating. There is no way out. The even mode
eventually will be convert back to the differential mode. Part of this even
mode energy will be radiated out. If the discontinuities are not very
serious, the radiated power should be minors and most of the energy will be
propagating as differential mode.

Regarding the question how the removal of the ground plane affect the Zc of
the differential mode, I do not know any analytical formula for it. I always
use full-wave electromagnetic simulator for it to get precise answer.
Thanks!

Best regards,

----------------------------------------------------------------------------
--

> -----Original Message-----
> From: Knighten, Jim L [mailto:JK100005@xxxxxxxxxxxxxxxxxxxxxxxxxxx]
> Sent: Friday, July 26, 2002 11:03 AM
> To: jian@xxxxxxxxxx; binchuan2000@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: RE: [SI-LIST] Re: Remove Ground underneath Differential signal
> is deserved or not?
>
>
> Dr. Zheng,
>
> I think it is more precise to say that you will suppress the even
> mode that
> flows on the ground plane.  Since the even mode is caused by
> characteristics
> of the signal (imbalance), it cannot be done away with.  It will
> find a path
> for propagation, just not on the ground plane.
>
> My two cents,
>
> Jim
>
-----------------------------------------------------------------------
Jian-X. Zheng, Ph.D
Zeland Software, Inc., 48890 Milmont Drive, 105D, Fremont, CA 94538, U.S.A.
Tel: 510-623-7162, Fax: 510-623-7135, Web: http://www.zeland.com
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>

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