Hello Jack,

When we build differential impedance boards we model the circuits as
pairs.  The model is different than single signal impedance models.
We will adjust the spacing/trace width/dielectric spacing to achieve
the correct impedance.  We also run differential testing on the
assigned signals on each layer (we run a signal test coupon for each
critical trace per layer) that are called out on the data.  If you
specify a circuit internally that requires testing we can test that
also.  This test is done after the board is completed.

Hope this helps,
Joe Dickson
Dir. of Engineering
Tyco Printed Circuits
Santa Clara Divsion





> 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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