Understanding Coplanar Waveguide over ground impedance

I am trying to understand why coplanar waveguide over ground does not have a lower impedance than microstrip, basically trying to get an idea how grou nd fill effects track impedance. Surely if the track/ground gap becomes lar ge then the circuit becomes a conventional Microstrip. My Saturns PCB microstrip calculator give 0.1mm track Z0 as 60R on outer la yer 0.1mm away from ground plane (36um copper), if I use Saturns PCB coplan ar waveguide over ground calculator it gives Z0 as 76R for 0.5mm clearance ground fill 0.1mm over ground plane, however as ground fill clearance incre ases impedance goes up which to me is illogical, if ground fill is a long w ay away it should have the same Z0 as microstrip. Can someone explain this to me? If I look at cross talk then this goes down the closer ground plane is to tracks and the further tracks are away from each other as you would expect. Thanks for any advice

Steve

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steve
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e a lower impedance than microstrip, basically trying to get an idea how gr ound fill effects track impedance. Surely if the track/ground gap becomes l arge then the circuit becomes a conventional Microstrip.

layer 0.1mm away from ground plane (36um copper), if I use Saturns PCB copl anar waveguide over ground calculator it gives Z0 as 76R for 0.5mm clearanc e ground fill 0.1mm over ground plane, however as ground fill clearance inc reases impedance goes up which to me is illogical, if ground fill is a long way away it should have the same Z0 as microstrip.

wn the closer ground plane is to tracks and the further tracks are away fro m each other as you would expect.

Try asking this on the si-list, there are many people with that sort of expertise there. I am sure you will get good answers.

Dimiter

------------------------------------------------------ Dimiter Popoff, TGI

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