RE: [SI-LIST] : Inner vs. Outer Layer Routing

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From: sweir (weirsp@a.crl.com)
Date: Wed Dec 22 1999 - 15:52:07 PST


Jian,

Thanks for taking the time to respond. It sounds like we are in pretty
good agreement on the basic physics. My postings were only in terms of
general theory and practice, not any attempt to solve a specific
problem. Thanks again for the clarifications.

Regards,

Steve.
At 02:04 PM 12/22/99 -0800, you wrote:
>Hi, Steve:
>
>I think I kind understand why you mean know. I also checked the original
>posting on the SI-LIST.
>
>Generally, it really depends. However, I am not sure what freqeuency range
>the applications is and what is the height from the strip to the top ground
>plane. It is also very critical to know whether there is any connection
>between the top ground plane to the bottom ground plane (infinite ground).
>
>If the top ground to strip distance is the same as the bottom ground to
>strip distance, if the top ground plane and the bottom ground plane are
>connected electrically, I believe the stripline case will generate much less
>radiation. It is by the nature of stripline. My comments on striplines in my
>last two postings are still valid. Whenever you have 3D interconnects in the
>signal lines, you should try to use some vias to connect the top ground and
>the bottom ground plane at the proximity of the 3D interconnects.
>
>Keeping the return path close to the signal path should be a good way to
>reduce the radiation no matter it is microstrip or strip line circuits. As
>somebody has mentioned, the fields from the two opposite currents will
>cancel each other.
>
>It is extremely important to have a good connection between the top ground
>plane and the bottom ground plane. Otherwise, the top ground plane will
>become an antenna radiating much of the power into the space. When you use
>the vias to connect the top ground plane and the bottom ground plane, you
>need to make sure the distance between two vias should be smaller than
>quarter a wavelength of the highest frequency in your circuit. If the
>distance between two adajcent vias is larger than quarter wavelength, you
>will see the resonance with strong power radiated.
>
>Such problems can be solved accurately and efficiently using our tools. It
>normally takes minutes to solve them. You can check our web site on it.
>Thanks!
>
>----------------------------------------------------------------------------
>------------
>Jian-X. Zheng, Ph.D
>Zeland Software, Inc., 39676 Mission Blvd., Fremont, CA 94539, U.S.A.
>Tel: 510-797-8109, Fax: 510-797-8241, Web: http://www.zeland.com
>----------------------------------------------------------------------------
>------------
>
> > -----Original Message-----
> > From: owner-si-list@silab.eng.sun.com
> > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of sweir
> > Sent: Tuesday, December 21, 1999 7:18 PM
> > To: si-list@silab.eng.sun.com
> > Subject: RE: [SI-LIST] : Inner vs. Outer Layer Routing
> >
> >
> > Jian,
> >
> > The assumption which I have made is that we have a multilayer
> > board in both
> > cases. So, in the microstrip case, we still have the problem
> > that when we
> > via, we leave the TEM mode as well. Without a matching via
> > between the two
> > ground planes, we cause considerable excitation in both:
> >
> >
> > ====| S
> > =============== G
> > |
> > ========|====== G
> > |==== S
> >
> > For this situation, I do not see that the stripline is
> > fundamentally better
> > or worse as the planes in both cases are subjected to substantial
> > pertubations. Do you agree that if we want to keep the EMI
> > down, we want
> > to stay in TEM as much as possible? My understanding of the
> > world is that
> > this means we can't let the field expand substantially into
> > dielectric. So, we need a Z axis return path to mirror the Z axis signal
> > path as closely as possible.
> >
> > If I am way off base, I welcome the education.
> >
> > Regards,
> >
> >
> > Steve.
> >
> >
> > At 03:01 PM 12/21/99 -0800, you wrote:
> > >Hi, Steve:
> > >
> > >I am not sure what "reflection plane to reflection plane vias"
> > mean in your
> > >message. Maybe it is defined earlier in this subject. I did not
> > get all the
> > >messages on this subject because my name was removed
> > accidentally from the
> > >SI-LIST due to a crash on the SI-LIST server. My last posted message is
> > >mainly an objection to the comment "stripline normally radiates more".
> > >Basically, this comment is normally true. However, there are exceptions.
> > >
> > >Regarding vias on microstrip circuits mentioned in your posting, the
> > >purposes for the vias in microstrip circuits may be different
> > from that for
> > >stripline circuit:
> > >
> > >A microstrip line has two metals: one for the strip and the other for the
> > >ground. There exist only one TEM or quasi-TEM mode on a
> > microstrip circuit.
> > >
> > >An illustration on microstrip
> > > ===== strip
> > >
> > >============== ground
> > >
> > >A stripline circuit has three metals: (1) the strip; (2) the top ground
> > >plane; (3) the bottom ground plane. We can excite a TEM or quasi-TEM mode
> > >between any two metals. However, we normally use the mode with
> > the excited
> > >between the strip (+ terminal) and the two ground planes (-
> > terminals). Any
> > >3D interconnects or holes on the ground planes may excite the
> > parallel plate
> > >mode. The parallel plate mode is between the two ground planes.
> > One of the
> > >ground planes becomes the positive terminal and the other become
> > negative.
> > >The parallel plate mode can be surpressed by building a via to
> > connect the
> > >top and bottom ground plane at the proximity of the vertical 3D
> > >interconnect. It is very effective.
> > >
> > >
> > >An illustration on strip line.
> > >
> > > ================ top ground
> > > === strip
> > > ================ bottom ground
> > >
> > >For microstrip structure, there is no parallel plate mode and we
> > do not need
> > >to use vias to surpress them. However, some people do try to use vias to
> > >suppress the surface wave mode, which is normally much less
> > serious than the
> > >parallel plate mode in stripline circuits. However, it is not very
> > >effective. In this sense, the vias in microstrip circuits and strip line
> > >circuits should serve different purposes. Thank you very much.
> > >
> > >Best regards,
> > >
> > >-----------------------------------------------------------------
> > -----------
> > >------------
> > >Jian-X. Zheng, Ph.D
> > >Zeland Software, Inc., 39676 Mission Blvd., Fremont, CA 94539, U.S.A.
> > >Tel: 510-797-8109, Fax: 510-797-8241, Web: http://www.zeland.com
> > >-----------------------------------------------------------------
> > -----------
> > >------------
> > > > -----Original Message-----
> > > > From: owner-si-list@silab.eng.sun.com
> > > > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of sweir
> > > > Sent: Tuesday, December 21, 1999 11:19 AM
> > > > To: si-list@silab.eng.sun.com
> > > > Subject: RE: [SI-LIST] : Inner vs. Outer Layer Routing
> > > >
> > > >
> > > > Jian,
> > > >
> > > > I believe that the need for reflection plane to reflection plane vias
> > > > proximate to high speed signal vias is the same for microstrip as
> > > > it is for
> > > > stripline.
> > > >
> > > > Regards,
> > > >
> > > >
> > > > Steve.
> > > >
> > > > At 12:57 PM 12/21/99 -0800, you wrote:
> > > > >Hi, everybody:
> > > > >
> > > > >Microstrip and stripline have their own advantage. Stripline
> > normally has
> > > > >less inductance and less dispersion. However, 3D interconnect in
> > > > stripline
> > > > >may create very strong radiation. Some person mentioned that
> > stripline
> > > > >radiates less. Such a conclusion is true when you have only the
> > > > horizontal
> > > > >straight lines or horizontal bends. However, if you have vertical
> > > > >interconnects in a stripline circuit, the vertical interconnects
> > > > may create
> > > > >very strong radiation. It is the so-called parallel plate
> > mode, and this
> > > > >mode does not have any cut off frequency. Normally, microwave
> > > > designers use
> > > > >shorting pin between the two ground planes at the proximity of 3D
> > > > >interconnects to suppress the parallel plate mode. Thanks!
> > > > >
> > > > >Best regards,
> > > > >
> > > > >-----------------------------------------------------------------
> > > > -----------
> > > > >------------
> > > > >Jian-X. Zheng, Ph.D
> > > > >Zeland Software, Inc., 39676 Mission Blvd., Fremont, CA 94539, U.S.A.
> > > > >Tel: 510-797-8109, Fax: 510-797-8241, Web: http://www.zeland.com
> > > > >-----------------------------------------------------------------
> > > > -----------
> > > > >------------
> > > > > >
> > > > >
> > > > >
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