If you have two symmetric transmission lines and you want to drive
them differentially, with no differential-mode reflections and ringing
appearing in the resulting load waveform, then Zdiff = 2 * Zodd is the
right resistor to hook across the two lines.
The sqrt(Zodd * Zeven) value is used in microwave design because it
has another magical property: it ensures zero forward crosstalk
between single-ended signals on a pair of symmetric transmission
lines. Take a look in the book by Pozar on Microwave Engineering for
an analysis of this case.
>>>>> "fabrizio" == fabrizio zanella <firstname.lastname@example.org> writes:
fabrizio> To calculate the differential impedance of a
fabrizio> transmission line pair, I've always used 1) Zdiff = 2 *
fabrizio> Zoo, where Zoo is the odd mode impedance of the pair.
fabrizio> This formula is used by the pc board manufacturers. I
fabrizio> was speaking to someone last week who has 30+ years in
fabrizio> the industry and said this formula is wrong, that the
fabrizio> real differential impedance should be calculated as 2)
fabrizio> Zdiff = square root of (Zoo * Zee), where Zee is the
fabrizio> even mode impedance.
fabrizio> If I compare 1) to 2), I get quite different values of
fabrizio> differential impedance. Does anyone have comments on
fabrizio> Thanks, Fabrizio Zanella EMC corporation
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