> There is a question I have had for several years; > I though I understood the answer but now recognize > that I don't. Perhaps some of our resident "Designer > Council" experts or some IPC gurus can help. > > The problem involves the formulas for intrinsic > capacitance of a trace, as published in IPC 317 and > 317A. For stripline, the formula for Co is: > Co=1.41Er/Ln[3.81H/(.8W+T)] (5.37 in 317A) > for Asymetric stripline, the formula is: > Co=2.82Er/Ln[2(H-T)/(.268W+.335T)] (5.38b in 317A) > > This later formula can be manipulated to > Co=2.82Er/[Ln[6(H-T)/(.8W+T)] > which can be compared with Eq. 5.37 > > Now assume a stripline case with > H = 10 (mils) > T = 1 oz > W = 8 > Er = 4.3 > > Co is calculated as 3.8 pf/in > > Now, simply move the lower plane very far away and use the > asymetric stripline formula. You get 6.42 pf/in, roughly > twice as much. But since you have only moved one plate > AWAY from the trace, intuitively you'd think the capacitance > would go DOWN by a factor of two, not up. > > Is there something wrong with my thinking, or with the > formulas????? > > Thanks for the help. > > Doug Brooks > . > **************************************************** > Doug Brooks, President [log in to unmask] > UltraCAD Design, Inc. http://www.ultracad.com > ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ################################################################