Ithemanz- The column you refer to relates to the signal ended impedance of a transmission line. Do not confuse this with the odd mode impedance of a transmission line that is part of a pair. The odd mode impedance of a line will decrease by as much as 10% as the second line in the pair is brought to a spacing equal to the line width. The odd mode impedance of a line is what influences the differential impedance of the pair. However, the single ended impedance of that line will not change as the second line is brought into proximity. If this is confusing, please see chapter 11 of my book, Signal Integrity Simplified, where I introduce the three different impedances each line has that is part of a pair: its single ended impedance, its odd mode impedance and its even mode impedance. These are basic and fundamental concepts about differential pairs. If you understand what these three terms mean, you will have a clear, intuitive understanding about the behavior of differential pairs. In general, the terms impedance and inductance, while perhaps the most important concepts in signal integrity, are also the most confusing. I think this is to large part because they refer to many different special cases that are very well defined, but we often do no distinguish which ones we are talking about in casual conversation. For example, with impedance, there is characteristic impedance, instantaneous impedance, input impedance, odd mode impedance, even mode impedance, and single ended impedance. Each has a very different and distinct meaning and applies to different cases. It is important to understand what each means. I am as guilty as everyone in not being as careful as I should in distinguishing which one of these I mean when I use the term impedance. Likewise, with inductance, there is self, mutual, partial, loop and total inductance, and combinations thereof. Each has a different, specific, well defined meaning that is perfectly consistent when used correctly. It's when we use just the term, "inductance" that much confusion can arise- which one do we really mean. I am preparing a lecture on the 4 most confusing concepts in signal integrity. These two terms are part of the list. If you have your favorite terms that are confusing or ambiguous, please pass them along to me, off line. Hope this helps. --eric ************************************** Dr. Eric Bogatin, President Bogatin Enterprises, LLC Setting the Standard for Signal Integrity Training 26235 w 110th terr Olathe, KS 66061 v: 913-393-1305 f: 913-393-0929 c:913-424-4333 e:eric@xxxxxxxxxxxxxxx www.BeTheSignal.com **************************************** -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of lthemanz Sent: Tuesday, August 21, 2007 1:29 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] diff pairs and spacing Hi folks, Typically our SI / EE guys tells us, give me a diff pairs with 1x or 1.5x spacing between +/-. If we apply below 'equation' "The proximity of the grounded second trace really does decrease the impedance of the signal trace, and it drops it more as the ground trace gets closer. However, the effect is very small. As long as the adjacent trace is at least a line width away, it has an impact of less than 0.1 on the impedance of the line. When it is within half a line width away, the impedance drops by 1 , and then drops faster as the ground trace gets closer." http://www.bethesignal.com/_FileLibrary/MonthlyColumn/64/BTS076_Calculating_ Characteristic_Impedence_0307PCDM.pdf Doesn't anything more then 1x have no proximity effect? Or am I applying this wrongly. thanks -lthemanz ____________________________________________________________________________ ________Ready for the edge of your seat? Check out tonight's top picks on Yahoo! 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