Dear Joe,

You might like to try Polar Instruments field solving
impedance calculator. This will let you see how
track seperation actually alters impedance on differential
traces.  You will find that the single end impedance of each
trace will need to be higher than usual as when driven differentially
the lines act as though a "virtual ground" or "ground wall" appears
between the two lines thus lowering the effective impedance.

Kind regards
Andy Burkhardt
www.polar.co.uk


At 08:20 18/03/99 -0500, you 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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