At 10:21 20/02/97 PST, you wrote:
> 
>I have seen a lot of information regarding striplines and microstrips,   
>but what about two traces routed as a pair without ground planes.  It is   
>quite common in a Printed Circuit Board that there is a need to route a   
>pair, like a tip and ring of a T1.  These should be a controlled   
>impedance to ensure minimize reflections,  these also need to be void of   
>ground or power planes to minimize noise and impedance to ground.  Does   
>there exist an equation to determine the impedance between two traces   
>without ground planes?  
>
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John,

Yes there are, but the maths is not simple.  The structure you are referring
to is a co-planar strip transmission line.  This may be constructed on a
single layer PCB or it is possible to add a ground plane.

The formula involves evaluation of Elliptic Integrals, but you may be able to
get an approximate correlation with the following simplified equation for a
parallel twin line (two wires side by side, wire radius = a, centre to centre
spacing distance = d, Dielectric constant of spacing medium = Er)

        Zo = (120/SQR(Er))*ln(d/a)

        where ln is natural log and d >> a

I would also suggest substituting Er' (Effective dielectric constant) for Er.
eg Er(AIR) = 1, Er(FR-4) = 4, so Er'(AIR&FR-4 combination) = 3 (approx)

A good indepth book on the subject is:
 
"Transmission Line Design Handbook", by Brian C. Wadell
 
1991 Artech House, Inc, ISBN 0-89006-436-9
 
         Artech House Publishers
         685 Canton Street               Tel: (617) 769-9750
         Norwood, MA 02062               Fax: (617) 769-6334
 
I hope this information helps.  Please email me if you require further
assistance.

Best regards

Andy Burkhardt <[log in to unmask]>
Tel: + 44 1481 53081
Fax: + 44 1481 52476
http://www.polar.co.uk
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World leaders in PCB faultfinding and controlled impedance measurement
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