RE: [SI-LIST] : Parallel single-ended traces routed to achieve 35 Ohm controlledimpedance

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Tue Oct 31 2000 - 12:46:06 PST


Ken, 35 and 70 ohms at the same time is not an unreasonable requirement.
Look at RAMBUS (28 ohms) and SCSI (>100 ohms). Sometimes you need to fit
the traces to meet the requirements of the application, not vice-versa.

Sal, I don't know if what you suggested has been tried before, but I think
it ought to work. To the extent that the two layers are isolated from one
another, you ought to get exactly half the impedance of each line alone.
Vias at intervals (much closer than a wavelength apart at your highest
harmonic frequency) might be good; but the higher in frequency you go, the
more vias are called for, yet the more they look like discontinuities.

However, for going through a pin field, instead of devising 35 ohm lines,
you probably can just neck down the traces. If the narrow portion is much
shorter than a wavelength (again), the impedance discontinuity would be
minor.

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