Let's Take A Vote...
in [Design]
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From: John Larkin on 27 Jul 2010 17:07 On Tue, 27 Jul 2010 15:06:19 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >John Larkin wrote: >> On Mon, 26 Jul 2010 19:42:04 -0700 (PDT), George Herold >> <gherold(a)teachspin.com> wrote: >> >>> On Jul 26, 2:55 pm, John Larkin >>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> On Mon, 26 Jul 2010 11:25:26 -0700 (PDT), George Herold >>>> >>>> >>>> >>>> >>>> >>>> <gher...(a)teachspin.com> wrote: >>>>> On Jul 26, 1:24 pm, John Larkin >>>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>>> On Sun, 25 Jul 2010 20:51:54 -0700 (PDT), George Herold >>>>>> <gher...(a)teachspin.com> wrote: >>>>>>> On Jul 25, 11:40 pm, John Larkin >>>>>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>>>>> On Sun, 25 Jul 2010 20:14:06 -0700 (PDT), George Herold >>>>>>>> <gher...(a)teachspin.com> wrote: >>>>>>>>> On Jul 25, 8:54 am, Phil Hobbs >>>>>>>>> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >>>>>>>>>> George Herold wrote: >>>>>>>>>>>>>> John >>>>>>>>>>>>>> [1] extra credit: how big would they be? >>>>>>>>>>>> Objects have both self-capacitance and mutual capacitance, so it's quite >>>>>>>>>>>> sensible to talk about a capacitor with only one lead. In Gaussian >>>>>>>>>>>> units, the self-capacitance of an isolated sphere of radius r >>>>>>>>>>>> centimetres is r. (The CGS unit of capacitance is the centimetre.) >>>>>>>>>>>> One cm ~= 1.12 pF, so 330,000 pF is about 30 km radius. That's quite a >>>>>>>>>>>> big reel! >>>>>>>>>>>> - Show quoted text - >>>>>>>>>>> (Or get Phil to check my math.) >>>>>>>>>>> George H. >>>>>>>>>> He's having enough trouble with his own recently--it took two tries this >>>>>>>>>> time. >>>>>>>>>> Cheers >>>>>>>>>> Phil Hobbs >>>>>>>>>> -- >>>>>>>>>> Dr Philip C D Hobbs >>>>>>>>>> Principal >>>>>>>>>> ElectroOptical Innovations >>>>>>>>>> 55 Orchard Rd >>>>>>>>>> Briarcliff Manor NY 10510 >>>>>>>>>> 845-480-2058 >>>>>>>>>> hobbs at electrooptical dot nethttp://electrooptical.net-Hidequotedtext - >>>>>>>>>> - Show quoted text - >>>>>>>>> My 4*pi was a guess. >>>>>>>>> What's more interesting is the C of an isolated trace with no ground >>>>>>>>> plane near by. (say some high impedance circuit) >>>>>>>>> Do you know how the C scales with the width? Assuming the length is >>>>>>>>> much greater than the width. >>>>>>>>> George H. >>>>>>>> Do you mean, like, a microstrip trace on an FR4 board with no ground >>>>>>>> plane anywhere? Like all such problems, it's messy. If the trace is >>>>>>>> narrow compared to the dielectric thickness, Er is midway between >>>>>>>> FR4's (around 4.6 maybe) and air. If the trace is much wider, Er >>>>>>>> approaches 1. >>>>>>>> I have tools to compute L and C per unit length for the common cases, >>>>>>>> microstrip with ground plane, stripline, CPW, things like that. Your >>>>>>>> case isn't among them. Easier to measure... if you can decide what to >>>>>>>> measure *to* >>>>>>>> I think Wadell's book covers that case, but his book is pretty much >>>>>>>> unusable. He has equations that cover a full page, and they include >>>>>>>> terms that themselves occupy other pages. >>>>>>>> John- Hide quoted text - >>>>>>>> - Show quoted text - >>>>>>> Oh, I was thinking about my question, ... Well first it should scale >>>>>>> with the length of the trace. (that's pretty obvious) And then I >>>>>>> thought there should be some logaritham(sp) of the width vs some other >>>>>>> distance... But I couldn't think what distance. It must be the >>>>>>> distance from the trace to where ever the nearest ground is... perhaps >>>>>>> the walls of the metal box enclosing it. >>>>>>> I wasn't thinking about the dielectric. That should be a secondary >>>>>>> effect... as long as the distance to the walls is a lot more than the >>>>>>> dielectric thickness. >>>>>>> George H. >>>>>> I checked: Wadell does a lot of weird cases, but not a conductor on >>>>>> dielectric and nothing else. His "covered microstrip" equation is 4 >>>>>> pages long! >>>>>> Look up ATLC, the free transmission-line calculator. It will solve >>>>>> cases like this. >>>>>> I think that a big grounded box will be the same as free space, as >>>>>> close as any of the tools can usefully resolve. >>>>>> John- Hide quoted text - >>>>>> - Show quoted text - >>>>> Thanks John, I'll see what I find. I really should just do the >>>>> problem for myself from a fundamental physics level. Assume an >>>>> infinite wire of radius R and calculate the capacitance per unit >>>>> length. >>>>> The 'real' question I have is, does it make sense to make really >>>>> skinny traces for a high impedance circuit with no ground plane? >>>>> Sense in that on want to keep the capacitance low. >>>> Absolutely. Use the shortest and skinniest traces you can, no planed >>>> nearby, no or tiny vias. >>>> >>>>> OK I got down the "Radio Engineers Handbook" by Terman, from my bosses >>>>> book shelf. (He's an old fart.) Terman does the case of a wire >>>>> diameter d a height h above a ground plane. For h>>d the capacitance >>>>> per foot (in units of micro micro Farads) >>>>> is, >>>>> C = 7.354/log(4h/d) >>>> Appcad (free from Agilent) does that case, but only gives you Zo and >>>> effective Er. I have a little PowerBasic program that converts those >>>> values to c and l per inch. You're welcome to it. >>>> >>>> Almost any simple equation, like the one above, gets inaccurate at >>>> certain geometries. The classic microstrip equation, like in the Moto >>>> ECL book, reports negative impedance for wide traces. Appcad is pretty >>>> good and will warn you when it isn't. As far as I can tell, many such >>>> equations are basically accidental curve fits, not based on much >>>> actual physics. >>> No, the logarithm is real physics. The E field between concentric >>> cylinders goes as 1/r. Integrating that gives a log of the ratio of >>> radii. (with a minus sign in the exponent.) >> >> Concentric infinitely-long cylinders is a case that has a closed-form >> solution. Finite parallel plates, wire over ground plane, microstrip, >> twisted pair, things like that probably don't. The equations you see >> are usually approximations with restrictions on geometry. >> >>> If I cut the width in half I only get some 30% improvement. Keeping >>> the traces as short as possible is much more important. (I think) >> >> Maybe fringing limits the improvement as you go skinnier. ATLC or >> Sonnet Lite would let you sim cases. I'd try it but I don't think I >> could accurately x-acto traces much skinnier than 15 mils maybe. >> >> John >> > >Geometries where the Laplacian separates, or 2-D geometries which can be >conformal-mapped into something nice, are about the only times you'll >get a closed-form solution. You need the conductor surfaces to be >curves of constant coordinate (e.g. the plane X=0 in Cartesian >coordinates or r=1 cm in cylindrical coordinates. I recall reading in >Morse and Feshbach that there are (iirc) only seven coordinate systems >in which the Laplacian separates, so the list of nice geometries is >fairly short. > >(The two wire problem can be conformal mapped to a parallel planes >problem, iirc, and you can use that to solve the wire-over-ground >problem by the method of images.) > >Cheers > >Phil Hobbs For what it's worth, for wire-over-ground-plane, Appcad complains if the separation is less than about 4 wire diameters, so if there is a closed-form solution, Agilent doesn't use it. I could see the small-separation case getting very messy. Wadell does some horrible cases, like Multiwire (enameled wire partially squished into dielectric) and wire above insulator with ground plane opposite, but they are ugly approximations. I just c-meter or TDR stuff like this, maybe do a few cases and graph it. John
From: George Herold on 27 Jul 2010 21:19 On Jul 27, 12:25 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Mon, 26 Jul 2010 19:42:04 -0700 (PDT), George Herold > > > > > > <gher...(a)teachspin.com> wrote: > >On Jul 26, 2:55 pm, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Mon, 26 Jul 2010 11:25:26 -0700 (PDT), George Herold > > >> <gher...(a)teachspin.com> wrote: > >> >On Jul 26, 1:24 pm, John Larkin > >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >> On Sun, 25 Jul 2010 20:51:54 -0700 (PDT), George Herold > > >> >> <gher...(a)teachspin.com> wrote: > >> >> >On Jul 25, 11:40 pm, John Larkin > >> >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >> >> On Sun, 25 Jul 2010 20:14:06 -0700 (PDT), George Herold > > >> >> >> <gher...(a)teachspin.com> wrote: > >> >> >> >On Jul 25, 8:54 am, Phil Hobbs > >> >> >> ><pcdhSpamMeSensel...(a)electrooptical.net> wrote: > >> >> >> >> George Herold wrote: > >> >> >> >> >>>> John > >> >> >> >> >>>> [1] extra credit: how big would they be? > >> >> >> >> >> Objects have both self-capacitance and mutual capacitance, so it's quite > >> >> >> >> >> sensible to talk about a capacitor with only one lead. In Gaussian > >> >> >> >> >> units, the self-capacitance of an isolated sphere of radius r > >> >> >> >> >> centimetres is r. (The CGS unit of capacitance is the centimetre.) > > >> >> >> >> >> One cm ~= 1.12 pF, so 330,000 pF is about 30 km radius. That's quite a > >> >> >> >> >> big reel! > > >> >> >> >> >> - Show quoted text - > > >> >> >> >> > (Or get Phil to check my math.) > > >> >> >> >> > George H. > > >> >> >> >> He's having enough trouble with his own recently--it took two tries this > >> >> >> >> time. > > >> >> >> >> Cheers > > >> >> >> >> Phil Hobbs > > >> >> >> >> -- > >> >> >> >> Dr Philip C D Hobbs > >> >> >> >> Principal > >> >> >> >> ElectroOptical Innovations > >> >> >> >> 55 Orchard Rd > >> >> >> >> Briarcliff Manor NY 10510 > >> >> >> >> 845-480-2058 > >> >> >> >> hobbs at electrooptical dot nethttp://electrooptical.net-Hidequotedtext- > > >> >> >> >> - Show quoted text - > > >> >> >> >My 4*pi was a guess. > >> >> >> >What's more interesting is the C of an isolated trace with no ground > >> >> >> >plane near by. (say some high impedance circuit) > >> >> >> >Do you know how the C scales with the width? Assuming the length is > >> >> >> >much greater than the width. > > >> >> >> >George H. > > >> >> >> Do you mean, like, a microstrip trace on an FR4 board with no ground > >> >> >> plane anywhere? Like all such problems, it's messy. If the trace is > >> >> >> narrow compared to the dielectric thickness, Er is midway between > >> >> >> FR4's (around 4.6 maybe) and air. If the trace is much wider, Er > >> >> >> approaches 1. > > >> >> >> I have tools to compute L and C per unit length for the common cases, > >> >> >> microstrip with ground plane, stripline, CPW, things like that. Your > >> >> >> case isn't among them. Easier to measure... if you can decide what to > >> >> >> measure *to* > > >> >> >> I think Wadell's book covers that case, but his book is pretty much > >> >> >> unusable. He has equations that cover a full page, and they include > >> >> >> terms that themselves occupy other pages. > > >> >> >> John- Hide quoted text - > > >> >> >> - Show quoted text - > > >> >> >Oh, I was thinking about my question, ... Well first it should scale > >> >> >with the length of the trace. (that's pretty obvious) And then I > >> >> >thought there should be some logaritham(sp) of the width vs some other > >> >> >distance... But I couldn't think what distance. It must be the > >> >> >distance from the trace to where ever the nearest ground is... perhaps > >> >> >the walls of the metal box enclosing it. > > >> >> >I wasn't thinking about the dielectric. That should be a secondary > >> >> >effect... as long as the distance to the walls is a lot more than the > >> >> >dielectric thickness. > > >> >> >George H. > > >> >> I checked: Wadell does a lot of weird cases, but not a conductor on > >> >> dielectric and nothing else. His "covered microstrip" equation is 4 > >> >> pages long! > > >> >> Look up ATLC, the free transmission-line calculator. It will solve > >> >> cases like this. > > >> >> I think that a big grounded box will be the same as free space, as > >> >> close as any of the tools can usefully resolve. > > >> >> John- Hide quoted text - > > >> >> - Show quoted text - > > >> >Thanks John, I'll see what I find. I really should just do the > >> >problem for myself from a fundamental physics level. Assume an > >> >infinite wire of radius R and calculate the capacitance per unit > >> >length. > > >> >The 'real' question I have is, does it make sense to make really > >> >skinny traces for a high impedance circuit with no ground plane? > >> >Sense in that on want to keep the capacitance low. > > >> Absolutely. Use the shortest and skinniest traces you can, no planed > >> nearby, no or tiny vias. > > >> >OK I got down the "Radio Engineers Handbook" by Terman, from my bosses > >> >book shelf. (He's an old fart.) Terman does the case of a wire > >> >diameter d a height h above a ground plane. For h>>d the capacitance > >> >per foot (in units of micro micro Farads) > >> >is, > >> >C = 7.354/log(4h/d) > > >> Appcad (free from Agilent) does that case, but only gives you Zo and > >> effective Er. I have a little PowerBasic program that converts those > >> values to c and l per inch. You're welcome to it. > > >> Almost any simple equation, like the one above, gets inaccurate at > >> certain geometries. The classic microstrip equation, like in the Moto > >> ECL book, reports negative impedance for wide traces. Appcad is pretty > >> good and will warn you when it isn't. As far as I can tell, many such > >> equations are basically accidental curve fits, not based on much > >> actual physics. > > >No, the logarithm is real physics. The E field between concentric > >cylinders goes as 1/r. Integrating that gives a log of the ratio of > >radii. (with a minus sign in the exponent.) > > Concentric infinitely-long cylinders is a case that has a closed-form > solution. Finite parallel plates, wire over ground plane, microstrip, > twisted pair, things like that probably don't. The equations you see > are usually approximations with restrictions on geometry. > > > > >If I cut the width in half I only get some 30% improvement. Keeping > >the traces as short as possible is much more important. (I think) > > Maybe fringing limits the improvement as you go skinnier. ATLC or > Sonnet Lite would let you sim cases. Hi John, I don't know any of those names, but don't waste any time doing this. The input circuit, I'm thinking about has all sorts of other stuff.. (switches and terminal blocks) hanging on the input and I 'think' the trace capacitance is minimal.... Ahh, the major contribution may be in the terminal block.?? Thanks? George H. I'd try it but I don't think I > could accurately x-acto traces much skinnier than 15 mils maybe. > > John- Hide quoted text - > > - Show quoted text -
From: George Herold on 27 Jul 2010 21:28 On Jul 27, 3:06 pm, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > John Larkin wrote: > > On Mon, 26 Jul 2010 19:42:04 -0700 (PDT), George Herold > > <gher...(a)teachspin.com> wrote: > > >> On Jul 26, 2:55 pm, John Larkin > >> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >>> On Mon, 26 Jul 2010 11:25:26 -0700 (PDT), George Herold > > >>> <gher...(a)teachspin.com> wrote: > >>>> On Jul 26, 1:24 pm, John Larkin > >>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >>>>> On Sun, 25 Jul 2010 20:51:54 -0700 (PDT), George Herold > >>>>> <gher...(a)teachspin.com> wrote: > >>>>>> On Jul 25, 11:40 pm, John Larkin > >>>>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >>>>>>> On Sun, 25 Jul 2010 20:14:06 -0700 (PDT), George Herold > >>>>>>> <gher...(a)teachspin.com> wrote: > >>>>>>>> On Jul 25, 8:54 am, Phil Hobbs > >>>>>>>> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > >>>>>>>>> George Herold wrote: > >>>>>>>>>>>>> John > >>>>>>>>>>>>> [1] extra credit: how big would they be? > >>>>>>>>>>> Objects have both self-capacitance and mutual capacitance, so it's quite > >>>>>>>>>>> sensible to talk about a capacitor with only one lead. In Gaussian > >>>>>>>>>>> units, the self-capacitance of an isolated sphere of radius r > >>>>>>>>>>> centimetres is r. (The CGS unit of capacitance is the centimetre.) > >>>>>>>>>>> One cm ~= 1.12 pF, so 330,000 pF is about 30 km radius. That's quite a > >>>>>>>>>>> big reel! > >>>>>>>>>>> - Show quoted text - > >>>>>>>>>> (Or get Phil to check my math.) > >>>>>>>>>> George H. > >>>>>>>>> He's having enough trouble with his own recently--it took two tries this > >>>>>>>>> time. > >>>>>>>>> Cheers > >>>>>>>>> Phil Hobbs > >>>>>>>>> -- > >>>>>>>>> Dr Philip C D Hobbs > >>>>>>>>> Principal > >>>>>>>>> ElectroOptical Innovations > >>>>>>>>> 55 Orchard Rd > >>>>>>>>> Briarcliff Manor NY 10510 > >>>>>>>>> 845-480-2058 > >>>>>>>>> hobbs at electrooptical dot nethttp://electrooptical.net-Hidequotedtext- > >>>>>>>>> - Show quoted text - > >>>>>>>> My 4*pi was a guess. > >>>>>>>> What's more interesting is the C of an isolated trace with no ground > >>>>>>>> plane near by. (say some high impedance circuit) > >>>>>>>> Do you know how the C scales with the width? Assuming the length is > >>>>>>>> much greater than the width. > >>>>>>>> George H. > >>>>>>> Do you mean, like, a microstrip trace on an FR4 board with no ground > >>>>>>> plane anywhere? Like all such problems, it's messy. If the trace is > >>>>>>> narrow compared to the dielectric thickness, Er is midway between > >>>>>>> FR4's (around 4.6 maybe) and air. If the trace is much wider, Er > >>>>>>> approaches 1. > >>>>>>> I have tools to compute L and C per unit length for the common cases, > >>>>>>> microstrip with ground plane, stripline, CPW, things like that. Your > >>>>>>> case isn't among them. Easier to measure... if you can decide what to > >>>>>>> measure *to* > >>>>>>> I think Wadell's book covers that case, but his book is pretty much > >>>>>>> unusable. He has equations that cover a full page, and they include > >>>>>>> terms that themselves occupy other pages. > >>>>>>> John- Hide quoted text - > >>>>>>> - Show quoted text - > >>>>>> Oh, I was thinking about my question, ... Well first it should scale > >>>>>> with the length of the trace. (that's pretty obvious) And then I > >>>>>> thought there should be some logaritham(sp) of the width vs some other > >>>>>> distance... But I couldn't think what distance. It must be the > >>>>>> distance from the trace to where ever the nearest ground is... perhaps > >>>>>> the walls of the metal box enclosing it. > >>>>>> I wasn't thinking about the dielectric. That should be a secondary > >>>>>> effect... as long as the distance to the walls is a lot more than the > >>>>>> dielectric thickness. > >>>>>> George H. > >>>>> I checked: Wadell does a lot of weird cases, but not a conductor on > >>>>> dielectric and nothing else. His "covered microstrip" equation is 4 > >>>>> pages long! > >>>>> Look up ATLC, the free transmission-line calculator. It will solve > >>>>> cases like this. > >>>>> I think that a big grounded box will be the same as free space, as > >>>>> close as any of the tools can usefully resolve. > >>>>> John- Hide quoted text - > >>>>> - Show quoted text - > >>>> Thanks John, I'll see what I find. I really should just do the > >>>> problem for myself from a fundamental physics level. Assume an > >>>> infinite wire of radius R and calculate the capacitance per unit > >>>> length. > >>>> The 'real' question I have is, does it make sense to make really > >>>> skinny traces for a high impedance circuit with no ground plane? > >>>> Sense in that on want to keep the capacitance low. > >>> Absolutely. Use the shortest and skinniest traces you can, no planed > >>> nearby, no or tiny vias. > > >>>> OK I got down the "Radio Engineers Handbook" by Terman, from my bosses > >>>> book shelf. (He's an old fart.) Terman does the case of a wire > >>>> diameter d a height h above a ground plane. For h>>d the capacitance > >>>> per foot (in units of micro micro Farads) > >>>> is, > >>>> C = 7.354/log(4h/d) > >>> Appcad (free from Agilent) does that case, but only gives you Zo and > >>> effective Er. I have a little PowerBasic program that converts those > >>> values to c and l per inch. You're welcome to it. > > >>> Almost any simple equation, like the one above, gets inaccurate at > >>> certain geometries. The classic microstrip equation, like in the Moto > >>> ECL book, reports negative impedance for wide traces. Appcad is pretty > >>> good and will warn you when it isn't. As far as I can tell, many such > >>> equations are basically accidental curve fits, not based on much > >>> actual physics. > >> No, the logarithm is real physics. The E field between concentric > >> cylinders goes as 1/r. Integrating that gives a log of the ratio of > >> radii. (with a minus sign in the exponent.) > > > Concentric infinitely-long cylinders is a case that has a closed-form > > solution. Finite parallel plates, wire over ground plane, microstrip, > > twisted pair, things like that probably don't. The equations you see > > are usually approximations with restrictions on geometry. > > >> If I cut the width in half I only get some 30% improvement. Keeping > >> the traces as short as possible is much more important. (I think) > > > Maybe fringing limits the improvement as you go skinnier. ATLC or > > Sonnet Lite would let you sim cases. I'd try it but I don't think I > > could accurately x-acto traces much skinnier than 15 mils maybe. > > > John > > Geometries where the Laplacian separates, or 2-D geometries which can be > conformal-mapped into something nice, are about the only times you'll > get a closed-form solution. You need the conductor surfaces to be > curves of constant coordinate (e.g. the plane X=0 in Cartesian > coordinates or r=1 cm in cylindrical coordinates. I recall reading in > Morse and Feshbach that there are (iirc) only seven coordinate systems > in which the Laplacian separates, so the list of nice geometries is > fairly short. > > (The two wire problem can be conformal mapped to a parallel planes > problem, iirc, and you can use that to solve the wire-over-ground > problem by the method of images.) Ahh, thanks Phil, you awaken memories from grad school. In any problem you can replace an equi-potential surface with a conductor. (The equi-potential surface at the mid-point between two wires is a plane.) George H. (this is either an example in Jackson or a problem.) > > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal > ElectroOptical Innovations > 55 Orchard Rd > Briarcliff Manor NY 10510 > 845-480-2058 > hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text - > > - Show quoted text -
From: George Herold on 27 Jul 2010 21:39 On Jul 27, 5:07 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Tue, 27 Jul 2010 15:06:19 -0400, Phil Hobbs > > > > > > <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > >John Larkin wrote: > >> On Mon, 26 Jul 2010 19:42:04 -0700 (PDT), George Herold > >> <gher...(a)teachspin.com> wrote: > > >>> On Jul 26, 2:55 pm, John Larkin > >>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >>>> On Mon, 26 Jul 2010 11:25:26 -0700 (PDT), George Herold > > >>>> <gher...(a)teachspin.com> wrote: > >>>>> On Jul 26, 1:24 pm, John Larkin > >>>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >>>>>> On Sun, 25 Jul 2010 20:51:54 -0700 (PDT), George Herold > >>>>>> <gher...(a)teachspin.com> wrote: > >>>>>>> On Jul 25, 11:40 pm, John Larkin > >>>>>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >>>>>>>> On Sun, 25 Jul 2010 20:14:06 -0700 (PDT), George Herold > >>>>>>>> <gher...(a)teachspin.com> wrote: > >>>>>>>>> On Jul 25, 8:54 am, Phil Hobbs > >>>>>>>>> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > >>>>>>>>>> George Herold wrote: > >>>>>>>>>>>>>> John > >>>>>>>>>>>>>> [1] extra credit: how big would they be? > >>>>>>>>>>>> Objects have both self-capacitance and mutual capacitance, so it's quite > >>>>>>>>>>>> sensible to talk about a capacitor with only one lead. In Gaussian > >>>>>>>>>>>> units, the self-capacitance of an isolated sphere of radius r > >>>>>>>>>>>> centimetres is r. (The CGS unit of capacitance is the centimetre.) > >>>>>>>>>>>> One cm ~= 1.12 pF, so 330,000 pF is about 30 km radius. That's quite a > >>>>>>>>>>>> big reel! > >>>>>>>>>>>> - Show quoted text - > >>>>>>>>>>> (Or get Phil to check my math.) > >>>>>>>>>>> George H. > >>>>>>>>>> He's having enough trouble with his own recently--it took two tries this > >>>>>>>>>> time. > >>>>>>>>>> Cheers > >>>>>>>>>> Phil Hobbs > >>>>>>>>>> -- > >>>>>>>>>> Dr Philip C D Hobbs > >>>>>>>>>> Principal > >>>>>>>>>> ElectroOptical Innovations > >>>>>>>>>> 55 Orchard Rd > >>>>>>>>>> Briarcliff Manor NY 10510 > >>>>>>>>>> 845-480-2058 > >>>>>>>>>> hobbs at electrooptical dot nethttp://electrooptical.net-Hidequotedtext- > >>>>>>>>>> - Show quoted text - > >>>>>>>>> My 4*pi was a guess. > >>>>>>>>> What's more interesting is the C of an isolated trace with no ground > >>>>>>>>> plane near by. (say some high impedance circuit) > >>>>>>>>> Do you know how the C scales with the width? Assuming the length is > >>>>>>>>> much greater than the width. > >>>>>>>>> George H. > >>>>>>>> Do you mean, like, a microstrip trace on an FR4 board with no ground > >>>>>>>> plane anywhere? Like all such problems, it's messy. If the trace is > >>>>>>>> narrow compared to the dielectric thickness, Er is midway between > >>>>>>>> FR4's (around 4.6 maybe) and air. If the trace is much wider, Er > >>>>>>>> approaches 1. > >>>>>>>> I have tools to compute L and C per unit length for the common cases, > >>>>>>>> microstrip with ground plane, stripline, CPW, things like that. Your > >>>>>>>> case isn't among them. Easier to measure... if you can decide what to > >>>>>>>> measure *to* > >>>>>>>> I think Wadell's book covers that case, but his book is pretty much > >>>>>>>> unusable. He has equations that cover a full page, and they include > >>>>>>>> terms that themselves occupy other pages. > >>>>>>>> John- Hide quoted text - > >>>>>>>> - Show quoted text - > >>>>>>> Oh, I was thinking about my question, ... Well first it should scale > >>>>>>> with the length of the trace. (that's pretty obvious) And then I > >>>>>>> thought there should be some logaritham(sp) of the width vs some other > >>>>>>> distance... But I couldn't think what distance. It must be the > >>>>>>> distance from the trace to where ever the nearest ground is... perhaps > >>>>>>> the walls of the metal box enclosing it. > >>>>>>> I wasn't thinking about the dielectric. That should be a secondary > >>>>>>> effect... as long as the distance to the walls is a lot more than the > >>>>>>> dielectric thickness. > >>>>>>> George H. > >>>>>> I checked: Wadell does a lot of weird cases, but not a conductor on > >>>>>> dielectric and nothing else. His "covered microstrip" equation is 4 > >>>>>> pages long! > >>>>>> Look up ATLC, the free transmission-line calculator. It will solve > >>>>>> cases like this. > >>>>>> I think that a big grounded box will be the same as free space, as > >>>>>> close as any of the tools can usefully resolve. > >>>>>> John- Hide quoted text - > >>>>>> - Show quoted text - > >>>>> Thanks John, I'll see what I find. I really should just do the > >>>>> problem for myself from a fundamental physics level. Assume an > >>>>> infinite wire of radius R and calculate the capacitance per unit > >>>>> length. > >>>>> The 'real' question I have is, does it make sense to make really > >>>>> skinny traces for a high impedance circuit with no ground plane? > >>>>> Sense in that on want to keep the capacitance low. > >>>> Absolutely. Use the shortest and skinniest traces you can, no planed > >>>> nearby, no or tiny vias. > > >>>>> OK I got down the "Radio Engineers Handbook" by Terman, from my bosses > >>>>> book shelf. (He's an old fart.) Terman does the case of a wire > >>>>> diameter d a height h above a ground plane. For h>>d the capacitance > >>>>> per foot (in units of micro micro Farads) > >>>>> is, > >>>>> C = 7.354/log(4h/d) > >>>> Appcad (free from Agilent) does that case, but only gives you Zo and > >>>> effective Er. I have a little PowerBasic program that converts those > >>>> values to c and l per inch. You're welcome to it. > > >>>> Almost any simple equation, like the one above, gets inaccurate at > >>>> certain geometries. The classic microstrip equation, like in the Moto > >>>> ECL book, reports negative impedance for wide traces. Appcad is pretty > >>>> good and will warn you when it isn't. As far as I can tell, many such > >>>> equations are basically accidental curve fits, not based on much > >>>> actual physics. > >>> No, the logarithm is real physics. The E field between concentric > >>> cylinders goes as 1/r. Integrating that gives a log of the ratio of > >>> radii. (with a minus sign in the exponent.) > > >> Concentric infinitely-long cylinders is a case that has a closed-form > >> solution. Finite parallel plates, wire over ground plane, microstrip, > >> twisted pair, things like that probably don't. The equations you see > >> are usually approximations with restrictions on geometry. > > >>> If I cut the width in half I only get some 30% improvement. Keeping > >>> the traces as short as possible is much more important. (I think) > > >> Maybe fringing limits the improvement as you go skinnier. ATLC or > >> Sonnet Lite would let you sim cases. I'd try it but I don't think I > >> could accurately x-acto traces much skinnier than 15 mils maybe. > > >> John > > >Geometries where the Laplacian separates, or 2-D geometries which can be > >conformal-mapped into something nice, are about the only times you'll > >get a closed-form solution. You need the conductor surfaces to be > >curves of constant coordinate (e.g. the plane X=0 in Cartesian > >coordinates or r=1 cm in cylindrical coordinates. I recall reading in > >Morse and Feshbach that there are (iirc) only seven coordinate systems > >in which the Laplacian separates, so the list of nice geometries is > >fairly short. > > >(The two wire problem can be conformal mapped to a parallel planes > >problem, iirc, and you can use that to solve the wire-over-ground > >problem by the method of images.) > > >Cheers > > >Phil Hobbs > > For what it's worth, for wire-over-ground-plane, Appcad complains if > the separation is less than about 4 wire diameters, so if there is a > closed-form solution, Agilent doesn't use it. I could see the > small-separation case getting very messy. Wadell does some horrible > cases, like Multiwire (enameled wire partially squished into > dielectric) and wire above insulator with ground plane opposite, but > they are ugly approximations. > > I just c-meter or TDR stuff like this, maybe do a few cases and graph > it. > > John- Hide quoted text - > > - Show quoted text - Thanks John, I didn't mean to get you all worked up. My input has ~7pF of capacitance. most of it in swithes and terminal blocks. (Hmm, that's good, our noise apparatus is shipping and I've been wondering what I'm going to be measuring to test them...) George H.
From: John Larkin on 27 Jul 2010 23:20
On Tue, 27 Jul 2010 18:19:30 -0700 (PDT), George Herold <gherold(a)teachspin.com> wrote: >On Jul 27, 12:25�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Mon, 26 Jul 2010 19:42:04 -0700 (PDT), George Herold >> >> >> >> >> >> <gher...(a)teachspin.com> wrote: >> >On Jul 26, 2:55�pm, John Larkin >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> On Mon, 26 Jul 2010 11:25:26 -0700 (PDT), George Herold >> >> >> <gher...(a)teachspin.com> wrote: >> >> >On Jul 26, 1:24�pm, John Larkin >> >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> >> On Sun, 25 Jul 2010 20:51:54 -0700 (PDT), George Herold >> >> >> >> <gher...(a)teachspin.com> wrote: >> >> >> >On Jul 25, 11:40�pm, John Larkin >> >> >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> >> >> On Sun, 25 Jul 2010 20:14:06 -0700 (PDT), George Herold >> >> >> >> >> <gher...(a)teachspin.com> wrote: >> >> >> >> >On Jul 25, 8:54�am, Phil Hobbs >> >> >> >> ><pcdhSpamMeSensel...(a)electrooptical.net> wrote: >> >> >> >> >> George Herold wrote: >> >> >> >> >> >>>> John >> >> >> >> >> >>>> [1] extra credit: how big would they be? >> >> >> >> >> >> Objects have both self-capacitance and mutual capacitance, so it's quite >> >> >> >> >> >> sensible to talk about a capacitor with only one lead. �In Gaussian >> >> >> >> >> >> units, the self-capacitance of an isolated sphere of radius r >> >> >> >> >> >> centimetres is r. �(The CGS unit of capacitance is the centimetre.) >> >> >> >> >> >> >> One cm ~= 1.12 pF, so 330,000 pF is about 30 km radius. �That's quite a >> >> >> >> >> >> big reel! >> >> >> >> >> >> >> - Show quoted text - >> >> >> >> >> >> > (Or get Phil to check my math.) >> >> >> >> >> >> > George H. >> >> >> >> >> >> He's having enough trouble with his own recently--it took two tries this >> >> >> >> >> time. >> >> >> >> >> >> Cheers >> >> >> >> >> >> Phil Hobbs >> >> >> >> >> >> -- >> >> >> >> >> Dr Philip C D Hobbs >> >> >> >> >> Principal >> >> >> >> >> ElectroOptical Innovations >> >> >> >> >> 55 Orchard Rd >> >> >> >> >> Briarcliff Manor NY 10510 >> >> >> >> >> 845-480-2058 >> >> >> >> >> hobbs at electrooptical dot nethttp://electrooptical.net-Hidequotedtext- >> >> >> >> >> >> - Show quoted text - >> >> >> >> >> >My 4*pi was a guess. >> >> >> >> >What's more interesting is the C of an isolated trace with no ground >> >> >> >> >plane near by. �(say some high impedance circuit) >> >> >> >> >Do you know how the C scales with the width? �Assuming the length is >> >> >> >> >much greater than the width. >> >> >> >> >> >George H. >> >> >> >> >> Do you mean, like, a microstrip trace on an FR4 board with no ground >> >> >> >> plane anywhere? Like all such problems, it's messy. If the trace is >> >> >> >> narrow compared to the dielectric thickness, Er is midway between >> >> >> >> FR4's (around 4.6 maybe) and air. If the trace is much wider, Er >> >> >> >> approaches 1. >> >> >> >> >> I have tools to compute L and C per unit length for the common cases, >> >> >> >> microstrip with ground plane, stripline, CPW, things like that. Your >> >> >> >> case isn't among them. Easier to measure... if you can decide what to >> >> >> >> measure *to* >> >> >> >> >> I think Wadell's book covers that case, but his book is pretty much >> >> >> >> unusable. He has equations that cover a full page, and they include >> >> >> >> terms that themselves occupy other pages. >> >> >> >> >> John- Hide quoted text - >> >> >> >> >> - Show quoted text - >> >> >> >> >Oh, I was thinking about my question, �... Well first it should scale >> >> >> >with the length of the trace. �(that's pretty obvious) �And then I >> >> >> >thought there should be some logaritham(sp) of the width vs some other >> >> >> >distance... But I couldn't think what distance. �It must be the >> >> >> >distance from the trace to where ever the nearest ground is... perhaps >> >> >> >the walls of the metal box enclosing it. >> >> >> >> >I wasn't thinking about the dielectric. �That should be a secondary >> >> >> >effect... as long as the distance to the walls is a lot more than the >> >> >> >dielectric thickness. >> >> >> >> >George H. >> >> >> >> I checked: Wadell does a lot of weird cases, but not a conductor on >> >> >> dielectric and nothing else. His "covered microstrip" equation is 4 >> >> >> pages long! >> >> >> >> Look up ATLC, the free transmission-line calculator. It will solve >> >> >> cases like this. >> >> >> >> I think that a big grounded box will be the same as free space, as >> >> >> close as any of the tools can usefully resolve. >> >> >> >> John- Hide quoted text - >> >> >> >> - Show quoted text - >> >> >> >Thanks John, �I'll see what I find. �I really should just do the >> >> >problem for myself from a fundamental physics level. �Assume an >> >> >infinite wire of radius R and calculate the capacitance per unit >> >> >length. >> >> >> >The 'real' question I have is, does it make sense to make really >> >> >skinny traces for a high impedance circuit with no ground plane? >> >> >Sense in that on want to keep the capacitance low. >> >> >> Absolutely. Use the shortest and skinniest traces you can, no planed >> >> nearby, no or tiny vias. >> >> >> >OK I got down the "Radio Engineers Handbook" by Terman, from my bosses >> >> >book shelf. (He's an old fart.) �Terman does the case of a wire >> >> >diameter d a height h above a ground plane. �For h>>d the capacitance >> >> >per foot (in units of micro micro Farads) >> >> >is, >> >> >C = 7.354/log(4h/d) >> >> >> Appcad (free from Agilent) does that case, but only gives you Zo and >> >> effective Er. I have a little PowerBasic program that converts those >> >> values to c and l per inch. You're welcome to it. >> >> >> Almost any simple equation, like the one above, gets inaccurate at >> >> certain geometries. The classic microstrip equation, like in the Moto >> >> ECL book, reports negative impedance for wide traces. Appcad is pretty >> >> good and will warn you when it isn't. As far as I can tell, many such >> >> equations are basically accidental curve fits, not based on much >> >> actual physics. >> >> >No, the logarithm is real physics. �The E field between concentric >> >cylinders goes as 1/r. �Integrating that gives a log of the ratio of >> >radii. �(with a minus sign in the exponent.) >> >> Concentric infinitely-long cylinders is a case that has a closed-form >> solution. Finite parallel plates, wire over ground plane, microstrip, >> twisted pair, things like that probably don't. The equations you see >> are usually approximations with restrictions on geometry. >> >> >> >> >If I cut the width in half I only get some 30% improvement. � Keeping >> >the traces as short as possible is much more important. �(I think) >> >> Maybe fringing limits the improvement as you go skinnier. ATLC or >> Sonnet Lite would let you sim cases. > >Hi John, I don't know any of those names, but don't waste any time >doing this. >The input circuit, I'm thinking about has all sorts of other stuff.. >(switches and terminal blocks) hanging on the input and I 'think' the >trace capacitance is minimal.... Ahh, the major contribution may be >in the terminal block.?? Thanks? > >George H. > > If you ever find time on your hands, play with this: http://atlc.sourceforge.net/ It's actually very clever. The core algorithms are impressively simple, and it makes neat color displays. I've used it for messy cases like calculating the impedances of buried (inner layer) microstrips and such. John |