Take the first of two DesignCon-related quizzes from the dean of Signal Integrity Academy.
With DesignCon 2015 coming in just 44 days, those of us who spend our time dealing with signal integrity problems are gearing up the the big event. With that in mind, I offer this week's Friday Quiz. I'll be back next week with another quiz covering S-Parameters.
Answers appear on the last page.
1. If you start with two, uncoupled, 50 Ω, microstrip transmission lines and just bring them closer so they are spaced equal to their line widths, which of the following is true?
The differential impedance drops below 50 Ω, and the common impedance increases above 50 Ω.
The odd mode impedance and even mode impedances stay the same.
The differential impedance drops below 90 Ω, and the common impedance increases above 30 Ω.
The differential impedance drops below 90 Ω, and the common impedance stays at 25 Ω.
The single-ended impedance stays the same, and the differential impedance increases.
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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.
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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