[SI-LIST] Impedance Uniformity
- From: "Mike Kauffman" <mkauffman@xxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 6 Oct 2003 15:49:48 -0700
Can anyone provide guidance on expected impedance variations along the
length of a differential grounded coplanar transmission line built with
foil construction?
I have made differential TDR measurements on several samples of a
nominally 100ohm, 10" long line on prepreg, and besides seeing a
difference in the average impedance, on some coupons, I see 'triangular'
variations of >10ohms occurring over 500ps of round trip delay. Another
sample, built with core construction showed no anomalous behavior - just
a slight rise of impedance that I attribute to conductor resistance.
I've been told by a board shop that for the outer layer, foil
construction is cheaper than core construction, so I'd like to avoid
core construction unless I have to.
Have others seen this behavior?
What specifications have you made on fab drawings to avoid it?
Thanks,
Mike Kauffman
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