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

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From: Scott McMorrow (scott@vasthorizons.com)
Date: Thu Nov 30 2000 - 14:15:04 PST


Larry,

You are correct.
For odd mode (differential) transmission, the centroid will be pulled in.
For even mode transmission, the centroid will be pushed out.

That's why I hate rules of thumb.
There is always an exception.

I would argue that anyone who is truly concerned about crosstalk
and all it's effects should invest time and money in performing
simulations. Beyond two lines and fairly simple structures,
the actual effects of crosstalk can be extremely complex, especially
in unterminated systems. (Noise, overshoot, undershoot, skew, jitter,
and effective change in propagation velocity and characteristic impedance
are all things that come to mind.)

regards,

scott

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

Larry Miller wrote:

> > > ....unless, of course, you are setting up a differential edge-coupled pair, in which case the cetroids of the fields will get > pulled off the centerlines of the traces. > > vasthorizons, eh? that must be a cocktail party conversation grabber.... <grin> > > Larry > > -----Original Message----- > From: Scott McMorrow [SMTP:scott@vasthorizons.com] > Sent: Thursday, November 30, 2000 9:00 AM > To: Loyer, Jeff W > Cc: si-list@silab.eng.sun.com > 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: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 > > > > **** To unsubscribe from si-list or si-list-digest: send e-mail to > > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE > > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. > > si-list archives are accessible at http://www.qsl.net/wb6tpu > > **** > > > > **** To unsubscribe from si-list or si-list-digest: send e-mail to > > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE > > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. > > si-list archives are accessible at http://www.qsl.net/wb6tpu > > **** > > -- > Scott McMorrow > Principal Engineer > SiQual, Signal Quality Engineering > 18735 SW Boones Ferry Road > Tualatin, OR 97062-3090 > (503) 885-1231 > http://www.siqual.com > > > **** To unsubscribe from si-list or si-list-digest: send e-mail to > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. > si-list archives are accessible at http://www.qsl.net/wb6tpu > **** >

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