Re: [SI-LIST] : Coplanar Transmission Line

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From: Chris Padilla (cpad@cisco.com)
Date: Thu Feb 03 2000 - 18:26:57 PST


A CPW may be composed with or without a ground plane--you'd better know
which so you use the proper model/formulas. They are often built on
structures meant for microstrips (hence the ground plane) since some
designs mix the two TR lines. So a CPW could have 2 or 3 metal
references.

----->Chris

At 05:51 PM, you wrote:
>
> Jian
>
> If you will check MDS or ADS you will find HP's definition by their models
> to be coplanar waveguide with or without a ground plane. No mention of
> coplanar strips. Since they stole their models and terminology from the
> origional research in the MTT Journals, I would tend to side with them.
>
> Ron Miller
>
> Jian Zheng wrote:
>>
>> 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,
>> ----------------------------------------------------------------------------
>>
>> ------------
>> Jian-X. Zheng, Ph.D
>> Zeland Software, Inc., Tel: 510-797-8109, Fax: 510-797-8241, Web:
>> <http://www.zeland.com>http://www.zeland.com
>> ----------------------------------------------------------------------------
>>
>> ------------
>>
>> > -----Original Message-----
>> > From: owner-si-list@silab.eng.sun.com
>> >
>> [<mailto:owner-si-list@silab.eng.sun.com>mailto:owner-si-list@silab.eng.s
>> un.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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>
>
> --
> Ronald B. Miller _\\|//_ Signal Integrity Engineer
> (408)487-8017 (' 0-0 ') fax(408)487-8017
> ==========0000-(_)0000===========
> Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA 95131
> rmiller@brocade.com, rbmiller@sjm.infi.net

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