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

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From: Doug Hopperstad (doug.hopperstad@qlogic.com)
Date: Thu Nov 30 2000 - 07:24:04 PST


Jeff,
Thanks for your comments regarding my question. The ratio of the trace to
plane (H) to the trace-to-trace spacing (D) is the fundamental factors in
determining the crosstalk. Yes there are other considerations but the most
popular formula regarding crosstalk is the listed in the Howard Johnson's
book. The formula is: 1/(1+(D/H)^2). This formula will provide the crosstalk
coefficient. I try to keep the value between 0.015 - 0.03. The other ratio,
D/H, should be kept to at least a 4:1 ratio. By using these values, the
amount of crosstalk will generally be small enough for most applications.

Your question regarding why you don't want to use S/H vs. D/W can be
answered this way: the closer you keep the trace to the ground plane the
better the coupling you will achieve. Try to keep the trace as wide as you
can. There are a few reasons for keeping the trace wide: 1. With a wider
trace, the effects of manufacturing tolerances and board trace edging will
be minimized, 2. The wider trace provides more coupling area to the ground
plane.

Doug Hopperstad
QLogic Corporation
doug.hopperstad@qlogic.com

-----Original Message-----
From: Loyer, Jeff W [mailto:jeff.w.loyer@intel.com]
Sent: Wednesday, November 29, 2000 2:55 PM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : D/W vs. S/H

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

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