Hi Joe.

        Just simply specifying the differential impedance requirements and
     the lines that are associated with that impedance is enough for fab
     shops that are capable of dealing with differential impedance.  You're
     right about wanting to avoid specifying the single ended impedance
     associated with the differential pair.  It is also significant to make
     the line width of the differential pairs different from any other
     single ended lines if any, just to allow for the likely adjustment of
     one without affecting the other.

     Mike Mikucki



______________________________ Reply Separator _________________________________
Subject: [TN] Specifying Differential Impedance
Author:  "Hurst; Joe" <[log in to unmask]> at smtplink-hadco
Date:    3/18/99 8:20 AM


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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