.MCAD 308000000 \  docDocumentMmcObject[ d2_graph_format graphData% axisFormat)L)Ltrace2D&&&&&&&&& & & & & &&& )D)D&&>1 1 1 1 NO-TRACE-STRING &1 1 1 1 NO-TRACE-STRING&1 1 1 1 NO-TRACE-STRING&1 1 1 1 NO-TRACE-STRING&1 1 1 1 NO-TRACE-STRING&1 1 1 1 NO-TRACE-STRING&1 1 1 1 NO-TRACE-STRING&1 1 1 1 NO-TRACE-STRING&1 1 1 1 NO-TRACE-STRING &1 1 1 1 NO-TRACE-STRING &1 1 1 1 NO-TRACE-STRING &1 1 1 1 NO-TRACE-STRING &1 1 1 1 NO-TRACE-STRING &1 1 1 1 NO-TRACE-STRING&1 1 1 1 NO-TRACE-STRING&1 1 1 1 NO-TRACE-STRING dim_formatTmasslengthtimecharge temperature luminosity substance)ent; "Shielded Coupled-Strip TransmissionNumericalFormatQdjj shpRectV0mcDocumentObjectState\ mcPageModelK????mcHeaderFooterI Narrow Edge Coupled Offset@I|D|F|P CHeaderFooterJ@{\rtf1\ansi\deff0\deftab720{\fonttbl{\f0\fswiss MS Sans Serif;}{\f1\froman\fcharset2 Symbol;}{\f2\fmodern\fprq3 Arial;}} {\colortbl\red0\green0\blue0;} \deflang1033\pard\plain\f2\fs18 \par } @{\rtf1\ansi\deff0\deftab720{\fonttbl{\f0\fswiss MS Sans Serif;}{\f1\froman\fcharset2 Symbol;}{\f2\fmodern\fprq3 Arial;}} {\colortbl\red0\green0\blue0;} \deflang1033\pard\qc\plain\f2\fs18 \{f\} \par } @{\rtf1\ansi\deff0\deftab720{\fonttbl{\f0\fswiss MS Sans Serif;}{\f1\froman\fcharset2 Symbol;}{\f2\fmodern\fprq3 Arial;}} {\colortbl\red0\green0\blue0;} \deflang1033\pard\qr\plain\f2\fs18 \{n\} of \{nn\} \par } @J@{\rtf1\ansi\deff0\deftab720{\fonttbl{\f0\fswiss MS Sans Serif;}{\f1\froman\fcharset2 Symbol;}{\f2\fmodern\fprq3 Arial;}{\f3\fswiss Arial;}} {\colortbl\red0\green0\blue0;} \deflang1033\pard\plain\f3\fs20 \par } @{\rtf1\ansi\deff0\deftab720{\fonttbl{\f0\fswiss MS Sans Serif;}{\f1\froman\fcharset2 Symbol;}{\f2\fmodern\fprq3 Arial;}{\f3\fswiss Arial;}} {\colortbl\red0\green0\blue0;} \deflang1033\pard\qc\plain\f3\fs20 Scott E. Farleigh \par } @{\rtf1\ansi\deff0\deftab720{\fonttbl{\f0\fswiss MS Sans Serif;}{\f1\froman\fcharset2 Symbol;}{\f2\fmodern\fprq3 Arial;}{\f3\fswiss Arial;}} {\colortbl\red0\green0\blue0;} \deflang1033\pard\qr\plain\f2\fs18 sef@uswest.net \par } @J@J{Gz?{Gz? TextState? TextStyle>@ Times New Roman0,0,255Serial_ParPropDefaultW?Normal>@ Symbol0,0,255@W?Symbolsfont_style_listO font_styleP  VariablesTimes New Roman@P  ConstantsTimes New Roman@P TextTimes New Roman@P Greek VariablesSymbol@P User^1Arial@P User^2 Courier New@P User^3System@P User^4Script@P User^5Roman@P User^6Modern@P User^7Times New Roman@P SymbolsSymbol@P Current Selection FontArial@P Undefined Font@P ,CHeaderArial@P ,CFooterArial@P Rotated Math FontTimes New Roman  TextRegion* docRegionGshpBoxU-(X CharacterMap-RangeMap;)BROADSIDE COUPLED STRIP TRANSMISSION LINE ChrPropMap7)  RangeElem<)  ChrPropData8 RangeData= | ParPropMap9) <)  ParPropData:@W,EmbedMap1<LinkMap/)<)LinkData0@ NormalTimes New Roman *@U4@-IoThis note assumes that the reader is already familiar with the Strip Transmission Line concept. For an introduction to Strip Transmission Lines please read Motorola MECL System Design Handbook, Motorola, or F100K ECL 300 Series Data Book and Design Guide, Fairchild Semiconductor. The following equations are from Cohn, "Characteristic Impedances of Broadside-Coupled Strip Transmission Lines" & "Thickness Corrections for Capacitive Obstacles and Strip Conductors", IEEE MTT, Nov. 1960. The equations, as implemented in this MathCad file, have been reduced from the equations in the two referenced IEEE article. The reduction of the equations are NOT obvious. One of the reasons for this is that Cohn uses different notation for the same variable between the two articles. Please note that these equations are accurate so long as the following inequality is met, . Given that the typical balanced impedance is 100 W and typical layer thicknesses, 5 mils and greater, the afore mentioned inequality will be satisfied. Also, the distance between the signal layer and the adjacent ground plane should be greater than the distance between the two signal layers. This will assure that signal in one signal conductor will couple more to the other signal conductor than to the ground plane. PWB vendors will complain that they cannot guarantee layer to layer registration so that two strips will lie directly atop each other. Most PWB vendors will claim that their limit of registration is 5 mils. This is adequate for this work. From other work the author has determined that this will only vary the characteristic impedance by about 3%. The drawback of mis-registration is that the coupling between the two strips will decrease as the strips become misaligned. To minimize misalignment the two signal layers should be etched on a common core, a.k.a. C stage. This will reduce, but not eliminate misalignment of the strips. In addition, PWB vendors will claim that these equations will not produce the desired results. These equations are correct for perfect variables. The PWB vendors have "tuned" their equations for their processes, i.e.. their equations take into account variances in their manufacturing process. Also, each vendor will have their own set of equations! The equations in this note will be very close to what the PWB vendor will call out for a specific impedance. Scott E. Farleigh sef@uswest.net7c$o <8<$8w<80@ NormalTimes New Roman ? BitmapRegion PictureRegionF@Ue@@ mc_dibitmap (X@A*@U@@@-@rA diagram of the cross-section of a Broadside Coupled Strip Transmission Line is shown in the Figure to the right.7r9r@B@@ A=A?@@dA>C.feA@@@A>AA@@tA@0.4413AB@@A@AC@@@ABAD@@tAC1AE@@AC\pAF@@pABAG@@AFAH@@@AGAI@@dAHlnAJ@@pAHAK@@AJAL@@dAKbAM@@pAKAN@@AMAO@@dANbAP@@ANsAQ@@AGAR@@@AQAS@@dARsAT@@pARAU@@ATAV@@dAUbAW@@AUsAX@@AQAY@@dAXlnAZ@@pAXA[@@AZA\@@dA[bA]@@A[sA^@B@U` $A_@@ pA`@@ A_Aa@@@A`Ab@@dAaZ.0eAc@@pAaAd@@AcwAe@@A`Af@@tAe188.3Ag@@AeAh@@{@AgAi@@Ah\e.rAj@@pAgAk@@AjAl@@@AkAm@@@AlAn@@dAmwAo@@AmbAp@@AlAq@@tAp1Ar@@ApAs@@dArsAt@@ArbAu@@AkC.feAv*@U4w@CC-AIn an earlier document; "Shielded Coupled-Strip Transmission Line", IEEE MTT, October 1955; Cohn states that "When a pair of strips are used as a balanced transmission line, the characteristic impedance Zb measured between (sic) the strips for the odd-mode distribution is the most (sic) useful quantity." This is related to Z0o by . That is, the balanced characteristic impedance is twice the odd-mode impedance.7MGAwEven Mode Impedence vs Trace Width &!Odd Mode Impedance vs Trace Width&Odd Mode as per Cohn&Differential as per Cohn&&&&& & & & & &&&