Re: [SI-LIST] : trace width for clock routing- wider/narrower?

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From: Scott McMorrow (scott@vasthorizons.com)
Date: Mon Mar 13 2000 - 11:27:31 PST


John,

For your examples of A) 5 mil tracks with 10 mil spacing
and B) 10 mil tracks with 20 mil spacing the following statements
are true, if the stackup is adjusted to give the same
characteristic impedance for each trace:

1) The crosstalk for case B will be lower by 1 or 2%.

2) DC attenuation will be twice as large for case A.

3) AC attenuation will be twice as large for case A.

4) Manufactured impedance control will be tighter for case B.

In crossing a split plane, if the traces are routed at the same
impedance then both will couple the same amount of energy
into the slot between the planes. Thus, both will have effectively
the same amount of emitted radation.

If the stackup is held constant, then impedance will be lower
for case B and crosstalk will be even lower for case B than
case A. The amount of emissions across a split plane will
depend upon the relative impedance matching of the slot
impedance and the microstrip impedance and the particular
frequency. For microstrip to slot transitions, there is no good
"rule of thumb" except to avoid them at all costs.

"Continuous copper planes are good."

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

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