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Microstip/CPW question - RF Cafe Forums

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boon

Post subject: Microstip/CPW question Posted: Thu Apr 13, 2006 7:21 pm

Captain

Joined: Tue Feb 21, 2006 5:19 pm

Posts: 17

hi

which one is less lossy: microstrips or CPW's given both are on the same substrate and the conductor metal is at the same distance away from the substrate? some discussion will be nice. thanks

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kpainter

Post subject: Posted: Thu Apr 13, 2006 10:56 pm

Colonel

Joined: Wed Aug 20, 2003 11:47 am

Posts: 47

Location: Santa Barbara, CA

IMHO and experience, the CPW is the less lossy of the two. This has more to do with the fact that the fields tend to be more contained than with Microstrip which go to infinity on the air side. The losses are due to radiation. Dielectric losses should be the same. Because the radiation is reduced, coupling between lines is also reduced. If you are worried about spurs, CPW is the way to go - that is unless you can do stripline.

Microstrip kinda sucks, otherwise folks wouldn't make antennas out of a similar structure.

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boon

Post subject: Posted: Mon Apr 17, 2006 6:17 pm

Captain

Joined: Tue Feb 21, 2006 5:19 pm

Posts: 17

thanks kpainter....but the field coupling to substrate is a major issue in on-chip transmission lines and wouldn't microstrip be better because the substrate is shielded by a ground plane whereas in CPW the conductor plane and ground plane are all on the same plane.

boon

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kpainter

Post subject: Posted: Tue Apr 18, 2006 1:25 am

Colonel

Joined: Wed Aug 20, 2003 11:47 am

Posts: 47

Location: Santa Barbara, CA

Hi Boon,

I assumed you were talking about grounded CPW. In this case, there is a ground below the substrate just like microstrip. It turns out, according to AWR's Txline program, that the calculated loss is approximately the same for grounded CPW and microstrip when the gap on the CPWG is the same as the transmission line width. Otherwise, the loss of CPWG is higher - which makes sense. I am saying that in my experience, the actual loss of microstrip is somewhat higher due to radiation. But then, I don't recall what gap I was using but I think it was equal to the line width.

In both structures, the primary propagation path is in the substrate, not in the air - otherwise Er would 1. I don't know how you could build the ungrounded version of CPW in a IC??

Then again, your results may vary.

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