Re: 1GB/s serial Diff Pairs over Backplanes and Connectors

Mike Jenkins (jenkins@lsil.com)
Tue, 30 Jul 1996 11:24:07 -0700

Chris,

Regarding your backplane transmission lines, there is some virtue
in constructing a 100 Ohm dif'l transmission line with thinner, more
closely spaced traces. Single-ended, the Zo is higher than 50 Ohms,
but, due to the coupling, the dif'l Zo can be 100 Ohms. (The dif'l
impedance is 2*[Zsingle-ended - Z12].)

The advantages are that tight coupling between the dif'l pair lines
1) decreases the probability of dif'l mode coupled noise, and 2) puts
more of the field lines in air above the FR-4, resulting in lower loss.
A disadvantage is the greater variation on Zo due to etch tolerance,
but starting at 15-20 mils leaves some room to play with. Also,
thicker dielectric between traces and ground plane helps this approach.

Regards,
Mike Jenkins

ps: LSI Logic has a Fibre Channel compliant CMOS CoreWare product also.
Evaluation boards are available running at 1.0625 Gbits, and a
number of our customers have applications very similar to yours.

-- 
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