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Return loss measurement

Title: Return loss measurement

Hi all,
When measuring return loss on the 2.5GB/s serial lines of PCI Express, are we required to include transmission line variation?  Are we allowed to just use 50ohms and call it a day? Otherwise you could have 20*log((60-40)/(60+40)) = 14dB which already violates the 15dB requirement for the receiver.

Bill Simms
Mixed Signal IC Designer
NVIDIA Corporation
(408)486-3318 (p)
(408)486-5718 (f)