Gary Ferrari wrote:
>
> Frank and Earl,
>
> Don't forget that if you must place construction characteristics such as line
> widths, and thicknesses on the drawing, make them for reference. The primary
> concern is that the impedance of the transmission line falls within the stated
> tolerance, not the individual features. Fabricators must tweak these features
> to match their process capabilities while meeting the final impedance
> requirement.
>
        Hi, all -

        I humbly submit that things change somewhat when an RF
        design includes etched components, such as the inductors
        that I think were mentioned.

        Rather than a nominal impedance, which is adequate for
        moderate speed digital signalling, IMHO this case needs
        a bit more invasive specification.  Etched components
        are too subject to the variations that will still all
        add up to the correct impedance, they are usually set
        to values of a component of impedance.  For better ref-
        erence, try to look for a "Smith Chart" to see how the
        multiple components of impedance converge.  HP has one
        online somewhere in a very good tutorial but I lack
        the link at the moment, sorry!

        It's a loop, really:  Contact the fabricator of choice
        and determine their optimimum values for the desired
        impedance.  Then use these values for determining the
        inductors, being sure to understand where/how the etch
        factors are being adjusted.  Then echo the fabricator's
        values, including laminate vendor and type #'s on the
        print.

        Good luck,
--

      Jeff Seeger                         Applied CAD Knowledge Inc
      Chief Technical Officer                  Tyngsboro, MA  01879
      jseeger "at" appliedcad "dot" com                978 649 9800

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