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August 1998

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Subject:
[Fab] Emperical Impedances
From:
George Franck <[log in to unmask]>
Reply To:
"TechNet E-Mail Forum." <[log in to unmask]>, George Franck" <[log in to unmask]>
Date:
Mon, 24 Aug 1998 08:50:17 -0400
Content-Type:
text/plain
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text/plain (45 lines) 
Technet,

Is it typical to have actual impedance reading which vary (10% or so) from
those predicted from theoritical data?

For example, in a cross section of a Stripline impedance coupon, I measured
the following dimensions.
========= Ground plane
========= 11.9 mil Dielectric (4.5 Dk)
========= Circuit Layer  1.2 mil thick,  9.0 mil wide
========= 11.8 mil Dielectric  (4.5 Dk)
========= Ground plane

Material is High Tg Epoxy.  The Dk is supplier estimate.  Theory would
predict a value of 49 Ohms.  Actual TDR measurements show an impedance of
54.7.

George Franck
Raytheon Systems
Falls Church Virginia
"My Opinions are just that."

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