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

From: Jian Zheng ([email protected])
Date: Tue Dec 21 1999 - 15:01:29 PST

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: [email protected]
> [mailto:[email protected]]On Behalf Of sweir
> Sent: Tuesday, December 21, 1999 11:19 AM
> To: [email protected]
> 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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