Can someone else please try to answer this question, too (nothing against
you, Raymond)
But differential pairs are a tricky area for us as well. Most people are
familiar with impedance control for single traces, but some fab shops don't
have decent software for pairs, and try to adjust trace widths incorrectly.
We are tempted to say "no modifications to artwork allowed" but I hate to do
that...

p.s. can anyone recommend the best impedance calculator (for the price) FOR
PAIRS?
We don't need some $20,000 package, just something decently reliable.

Jack


                -----Original Message-----
                From:   Raymond Aguilar [mailto:[log in to unmask]]
                Sent:   Wednesday, April 07, 1999 8:26 PM
                Subject:        Re: Specifying Differential Impedance

                Aloha Joe ,

                On design where I have to be concern with impedance I
typically  specify the trace width with a tolerance
                and the layer it is on and flag note it on the fabrication
drawing as a impedance controlled item.

                Aloha Ray

                "Hurst, Joe" wrote:

                > Hi
                >
                >         I have some very important differential pairs on a
PWB that must
                > have a particular differential impedance.
                >
                >         The problem is:  how can I specify this on the
drawing sent to the
                > fab shop?
                >
                >         Our usual method would be to call for a particular
single-ended
                > impedance, which seems to be beating around the bush.
It work to a
                > point; but I think it leaves too many variables that could
change the
                > differential impedance.
                >
                >         Any thoughts on this?
                >
                > Joe Hurst
                > PWB Designer
                > Computing Devices Canada
                > A General Dynamics Company
                > [log in to unmask]
                > 613-596-7816
                >
                >
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