# [SI-LIST] Re: Diff.Pairs

*From*: MikonCons@xxxxxxx*To*: bmgman@xxxxxxxxxx, scott@xxxxxxxxxxxxx*Date*: Mon, 13 Oct 2003 17:48:04 EDT

Scott & Mike: You guys definitely have the right idea(s). One thing you forgot to mention, or that others on this thread might not have thought of, is that a reference plane between the traces of a "differential" pair provides excellent decoupling/isolation of signals on the two traces. In addition to the variations in effective dielectric constant (and its negative impacts on skew and even-mode impedance), the plane provides electrical isolation between the top and bottom surface plane currents via the skin effect. Even a 1/2-ounce copper plane would provide 26 dB reduction in magnitude of the image plane return current from one side of the plane to the other at only 120 MHz. The isolation is virtually complete for the GHz data rates that this thread addresses. I echo Scott's disdain for any high-speed design for differential pairs to use this approach. RE: Scott's comment below, "For vias spaced larger than 150 mils away from a ground via (most signal vias), there is about -0.8 dB insertion loss at 3 GHz. This is comparable to a an 8.8% reduction in the signal amplitude of a 100 ps edge. At 7 GHz (50 ps rise time) there is a -1.5 dB loss (16% amplitude reduction.) These may or may not be significant losses in a design, but are extremely significant to the noise and EMI profile of a board, since all of this energy is being injected into the planes where it is not well contained. The implication for 2.5, 3.125 and 10 Gbps PCB designs is profound. Decouple the differential vias and get ready for increased losses, increased noise and increased EMI." I couldn't agree more. As supporting evidence over the last four years, I was the third (and only successful) consultant to analyze and define routing rules for multi-GHz boards and backplanes for Nokia Networking. I found it necessary to use closely-coupled, coplanar pairs in open field routing, and widened, symmetrical traces within pin fields that bracketed other differential pairs (to obtain canceling common-mode coupling fromn each +/- pair bracketed). My first guidance was to NOT use vias if at all possible. For unavoidable conditions, I devised a via structure that widened the narrowly spaced pair traces and bracketed two ground vias before coming back to close spacing on the new coplanar routing layer. The structure created a vertical two (round) wire pair of 100 Ohms differential impedance for minimal signal disturbance at the layer transition. Other consultants had tried multiple loosely-coupled techniques for 1.5 years without acceptable results (names withheld as they are too recognizeable). In other words, tight coupling was the only viable solution, along with carefully designed via-pair transitions. Keep up the good comments for the less experienced designers out there! Good thoughts also for Wyland and Knighten contributions. It's 70 degrees, sunny, and the trout are rising. Bye. Mike Michael L. Conn Owner/Principal Consultant Mikon Consulting *** Serving Your Needs with Technical Excellence *** ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: http://www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu

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