>One was that FR-4 material is made by a variety
>of vendors, and it is not all the same. A fab
>house will be taking those differences into account
>for their controlled impedance calculations,
>depending on the material vendor they use.
That's what my vendors tell me also.
I often have a lot of lumped loads, which have considerably
more capacitance than the trace between them. In those cases
I opt for fixed separation, regardless or Er which gives me repeatable
inductance between my lumped loads.
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