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Friday Quiz: Differential Pairs

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bogatin
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Re: Question 3
bogatin   12/15/2014 8:52:34 AM
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The link to the EPSI lesson, while an interesteing lesson about diff pairs, isn't the one I was thinking of. It does not talk about the important role of the adjacent plane.

This is the lesson, in the Advanced Gigabit Channel Design Course that discusses the role of the adjacent plane in differential pairs.

bogatin
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Re: Question 3
bogatin   12/15/2014 8:45:48 AM
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Bert-  You are correct that the obvious answer is the correct answer. Contrary to what is commonly reported in the popular lierature, cross talk between adjacent differential pairs in stripline is NOT affected by coupling. 

This can be easily shown using a simple 2D field solver.

In addition, if you keep the line width fixed and the differential impedance fixed, tighter coupled stripline differential pairs have MORE channel to channel cross talk than loosely coupled pairs.

However, when you have differential pairs WITHOUT an adjacent return plane, then it is correct that tighter coupling will decrease cross talk. This is in some connectors, in diff vias, in ribbon cable, in USTW cables. 

We have to be careful making sweeping generalizations about diff pairs. I cover this topic in a lesson in the EPSI course on the Signal Integrity Academy. 

Bert22306
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Question 3
Bert22306   12/12/2014 5:46:48 PM
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Aside from the fairly obvious advantage of achieving higher interconnect density, doesn't tight coupling in differential transmission lines also result in better rejection of noise, which would manifest as common mode signal? Pretty positive the answer is yes. So I picked b, "Reduces cross talk to external noise," even though I would not have phrase it that way.

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