# RE: [SI-LIST] : D/W vs. S/H

From: [email protected]
Date: Thu Nov 30 2000 - 15:40:05 PST

Jeff and Scott,
To get back to the original question of "why we wouldn't specify S/H instead
of D/W" in the context of crosstalk...

When the issues is crosstalk, the two systems (S/H and D/W) are somewhat
equivalent ratios. Once you've chosen an impedance, you can't change H
without changing W. IE, increasing H forces an increase in W. Only thing
left is trace to trace separation which can be specified with either edge to
edge or center to center method. You could also develop S/W or D/H systems.
I think we all agree that increasing S or D reduces crosstalk. The only
reason to prefer one system over another is "rule or thumb" familarity with
a particular set of numbers. And if you need more accuracy than "rule or
thumb" you must simulate. Can anyone show data that shows otherwise?

Aubrey Sparkman
Signal Integrity
[email protected]
(512) 723-3592

> -----Original Message-----
> From: Scott McMorrow [mailto:[email protected]]
> Sent: Thursday, November 30, 2000 11:00 AM
> To: Loyer, Jeff W
> Cc: [email protected]
> Subject: Re: [SI-LIST] : D/W vs. S/H
>
>
> Jeff,
>
> You are correct. As you and Larry Miller
> have pointed out, it is the ratio of S/H that is
> critical parameter for crosstalk. However, in order to
> execute the layout, the SI engineer will want to
> translate this into a minimum S for use by the
> layout team.
>
> And actually, to be more correct, the ratio that is
> truly important is D/H. That is, the center
> to center spacing of the traces divided by the distance
> to the plane. The center of the trace is the center
> of moment of the created fields. A rule of thumb
> based upon D will more accurately scale across
> various trace widths.
>
> scott
>
>
>
> "Loyer, Jeff W" wrote:
>
> > Doug's query brought up a related question to my feeble mind...
> >
> > Is there any reason to specify distance between traces
> relative to their
> > width? As far as I know, the most critical dimensions to
> consider are: 1)
> > distance between the edges of two traces, relative to 2)
> distance between
> > the trace and its ground plane(s). The width of the
> conductor is not a
> > significant factor, unless you're using center-to-center
> separation, where
> > you'll have to take into account the width. I don't
> understand why we
> > wouldn't specify S/H instead of D/W (see below).
> >
> > ______________________________________________________ GND
> > ^
> > |
> > (H)
> > |
> > v
> > ___________ <--- (S) ---> ___________ Signals traces
> > <-- (W) -->
> > <---------- (D) ---------->
> >
> > Jeff Loyer
> > (253) 371-8093
> >
> > -----Original Message-----
> > From: Doug Hopperstad [mailto:[email protected]]
> > Sent: Wednesday, November 29, 2000 12:27 PM
> > To: [email protected]
> > Subject: [SI-LIST] : RE: Crosstalk Bus spacing
> >
> > When determining the minimum spacing between traces on a
> digital bus, is it
> > best to setup the three traces as follows:(The design is
> using a stripline)
> >
> > "A": Aggressor trace
> > "V": Victim trace
> > "A": Aggressor trace
> >
> > -------------------------------------------------------
> Ground Plane layer
> > ------(A)------ ------(V)------ ------(A)------ Trace
> layer, 0.5
> > ounce.
> > -------------------------------------------------------
> Ground Plane layer
> >
> > Should both Aggressors be in-phase with each other or
> should one of them be
> > inverted to get the worst case crosstalk. I am simulating with both
> > applications and getting much more crosstalk on the victim
> trace when both
> > aggressors are in-phase.
> >
> > The clock edge rate is 950pS and the trace width is set at
> w = 5 mils. The
> > Plane to trace layer spacing is 6.5 mils. This provides a
> nice 50 ohm trace
> > impedance.
> > The distance between traces is set at 5 mils (1w). I have
> been playing with
> > 2w in the simulations as well.
> >
> > Is it traditional to set the trace-to-trace spacing on the
> bus traces, i.e.
> > bits(0:x) for example, at 1w the trace width. The
> > have been set for 2w spacing. The clock spacing is set for
> a 3w minimum.
> >
> >
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> --
> Scott McMorrow
> Principal Engineer
> SiQual, Signal Quality Engineering
> 18735 SW Boones Ferry Road
> Tualatin, OR 97062-3090
> (503) 885-1231
> http://www.siqual.com
>
>
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