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Reply To: | TechNet E-Mail Forum. |
Date: | Fri, 8 Jan 1999 19:13:55 -0500 |
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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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