Jack
The exact form is up to you, but we would typically want to see
something along these lines:
100 ohm differential impedance on layer 1 reference layer 2.
Trace/space/trace is 8/7/8.
You could also specify the impedance as "50 ohm odd-mode differential"
since that is what we would measure.
As for controlling modifications by fabricators to your linewidth, I
think it depends on the situation. We typically prefer to be given the
option to adjust dielectric thickness and conductor trace/space to
achieve the desired impedance value. We also prefer to work up front
with the designer to verify that the design is compatible with the
desired values. We also have plenty of customers that forbid
modifications without approval. One reason I can see to not allow
modifications on differentials is the interaction between
trace/space/trace. Using the example of 8/7/8 above, what if it needs
to be adjusted to 7/8/7? The instructions for Tooling say "change the 8
mil traces to 7 mils". Do you get 7/8/7 or 7/7/7? I've seen it happen.
For modelling, I've been happy with Apsim's RLGC and it will meet your
cost objective. You can find out more at
http://www.apsimtech.com/RLGC.htm
It's a 2-D field solver. It's flexible. Accurate (with accurate
inputs). One drawback is it doesn't have the nice user interface I've
seen on some other field solvers and simulators, but it's also not as
expensive.
> ----------
> From: Jack Olson[SMTP:[log in to unmask]]
> Reply To: TechNet E-Mail Forum.;Jack Olson
> Sent: Thursday, April 08, 1999 9:16 AM
> To: [log in to unmask]
> Subject: Re: [TN] Specifying DIFFERENTIAL Impedance
>
> 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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