RE: [SI-LIST] : RE: Crosstalk Bus spacing

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From: Larry Miller (ldmiller@nortelnetworks.com)
Date: Wed Nov 29 2000 - 18:59:27 PST


With the aggressor signals out of phase you have a null (cancellation) at
the victim site.
You can look at it as having a virtual ground there. So all aggressors
should be in phase for worst case, or maybe random phase for RMS.

What IS critical you do not show: the ratio of distance between traces to
the distance to the nearest return current plane(s). In your sketch it looks
like that might be the upper plane if the two ground planes are connected
together, but that may just be a type font artifact.

If you set the distance to the ground(s) to get a particular impedance (say
50 ohms, typical-- go visit the www.ultracad.com site to get a free
calculator to download), let's say that the distance to a ground is D. Then,
in general, you can use 3D as trace-to-trace spacing and have very low
crosstalk. (As your clock trace calculation evidently showed.)

You can also download a free calculator for crosstalk at ultracad. I am not
trying to beat the ultracad drum, but I have found these to be very handy
for just the kind of problem you are trying to solve. However it sounds like
you have some way of estimating crosstalk already.

Larry Miller

> -----Original Message-----
> From: Doug Hopperstad [SMTP:doug.hopperstad@qlogic.com]
> Sent: Wednesday, November 29, 2000 12:27 PM
> To: si-list@silab.eng.sun.com
> 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.
>
> Doug Hopperstad
>
>
>
>
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