RE: [SI-LIST] : 50 or 60 ohm impedance

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From: Zabinski, Patrick J. (zabinski.patrick@mayo.edu)
Date: Tue Dec 12 2000 - 14:55:35 PST


> One problem with using 50 ohms across the
> board is you
> often lose some routing density, if you stack up requires say
> 5 mil traces
> to get 50 ohms.

Not necessarily.

Take your favorite EM tool and simulate a pair of 50 ohm
traces at a given pitch, say 15 mils. Then, using the same
dielectric sandwich, narrow the traces a bit to obtain
60 ohms, and simulate with the same pitch (e.g., 15 mils).

What you'll see (hopefully) is that the crosstalk is less
for the 50 ohm traces than the 60 ohm traces for a given
pitch. This in turn means you can use a tighter pitch
for the 50 ohm traces than the 60 ohm traces.

Pat

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