[SI-LIST] : Differential impedance

fabrizio zanella (fabrizio=zanella%eng%emchop1@fishbowl02.lss.emc.com)
Wed, 11 Nov 98 19:04:15 -0500

To calculate the differential impedance of a transmission line pair, I've
always used
1) Zdiff = 2 * Zoo, where Zoo is the odd mode impedance of the pair. This
formula is used by the pc board manufacturers.
I was speaking to someone last week who has 30+ years in the industry and
said this formula is wrong, that the real differential impedance should be
calculated as
2) Zdiff = square root of (Zoo * Zee), where Zee is the even mode
impedance.

If I compare 1) to 2), I get quite different values of differential
impedance. Does anyone have comments on this?

Thanks, Fabrizio Zanella
EMC corporation
fzanella@emc.com

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