RE: [SI-LIST] : Coplanar Transmission Line

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From: Jian Zheng (jian@zeland.com)
Date: Thu Feb 03 2000 - 12:38:48 PST


Hi,

I noticed that some people in the SI list consider two coupled strips
(without ground plane) as co-planar waveguide. However, in the microwave
terminology, two coupled strips without ground plane is called co-planar
strips (or CPS). Three strips with two gaps are normally called co-planar
waveguide (CPW).

Compared to normal microstrip structures (strip over a substrate over a
ground plane), both CPS or CPW may have less dielectirc loss. The reason is
that about 50% of the field will be in the air and dielectric loss is less
important. For microstrip, about 60-95% of the field will be in the
substrate and the dielectric loss is higher. As metallic loss (loss in the
metal), CPW normally has less loss too because its metallic path is normally
wider. It is hard to say for CPS because it really depends upon how wide the
strip is.

Some clear advantage of CPW and CPS over microstrip are:

(1). CPW and CPS have less dispersion than microstrip. In the other word,
their equivalent circuit parameters (LRCG) are more constant over a wide
frequency range.

(2). Another advantage is that it is easier to achieve short circuit which
are common in microwave: On microstrip, you have to build the via holes for
the short circuit. On CPW and CPS, you can build them on the surfaces.

One important issue of CPW is the balance of the mode(s). Theoretically,
there is one fundamental mode (cut off frequency at DC) for a two-conductor
transmission line system. There are multiple fundamental modes for a
3-conductor system. For microstrip and CPS, there are two conductors. Only
1-fundamental mode exists. As long as the connectors are far away from
discontinuities (microwave terminology for interconnects of different
cross-section shape in the transmission line), only one single balanced
differential mode can exist. For CPW, there are 3 conductors. There are
multiple fundamental modes: (1) -1 1 -1; (2) -1 0 1; (3) -1 -1 1....
Normally, people use the 1st fundamental mode. However, any discontinuities
such as a bend can induce the other fundamental modes. People normally use
wire bonds to connect the -1 conductors to suppress the other higher order
modes.

I am not sure where you can find text books for this topic. Thanks!

Best regards,
----------------------------------------------------------------------------
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Jian-X. Zheng, Ph.D
Zeland Software, Inc., Tel: 510-797-8109, Fax: 510-797-8241, Web:
http://www.zeland.com
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> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Roy Leventhal
> Sent: Thursday, February 03, 2000 10:58 AM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : Coplanar Transmission Line
>
>
>
>
> Dear SI Experts,
>
> Who out there has worked with coplanar (waveguide-like)
> transmission lines as
> opposed to stripline/microstripline?
>
> What can you tell me about it other than what I can find in
> Collins or Bahl &
> Bhartia?
>
>
> Thanks in advance,
>
>
> Roy
>
>
>
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