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

From: Scott McMorrow ([email protected])
Date: Thu Nov 30 2000 - 09:00:29 PST

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 bus-to-adjacent traces
> 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