Let's Take A Vote...
in [Design]
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From: Jim Thompson on 26 Jul 2010 12:57 On Sun, 25 Jul 2010 18:09:32 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >On Sun, 25 Jul 2010 20:04:07 -0500, John Fields ><jfields(a)austininstruments.com> wrote: > >>On Sun, 25 Jul 2010 12:40:12 -0700, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Sun, 25 Jul 2010 13:26:21 -0500, John Fields >>><jfields(a)austininstruments.com> wrote: >>> [snip] >>>> >>>>"The Moving Finger writes; and, having writ, >>>> Moves on: nor all thy Piety nor Wit >>>>Shall lure it back to cancel half a Line, >>>> Nor all thy Tears wash out a Word of it." >>>> >>>> >>>>From Edward Fitzgeralds's translation of the >>>>Rub�iy�t of Omar Khayy�m. >>> >>>I guess you didn't read the wiki article. I pointed you at the easiest >>>part. >>> >>>http://en.wikipedia.org/wiki/Dimensional_analysis#Commensurability >> >>--- >>I read it, but it seems you're trying to prove whatever your point is >>by grasping at straws. >> >>Can the force between like charges not be determined by the magnitude >>of the charge(s) and the separation between them? >> >>JF > >Careful there John Fields, Phil Hobbs will plonk you... like you could >care :-) > >I think I still have the whole thread in "Trash", since I haven't >emptied it for awhile. I'll track down the "force" discussion. > > ...Jim Thompson John F, I've just E-mailed you the portions of the thread that contain the word "force", with headers. So you can peruse it and find what you need. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Spice is like a sports car... Only as good as the person behind the wheel.
From: John Larkin on 26 Jul 2010 13:24 On Sun, 25 Jul 2010 20:51:54 -0700 (PDT), George Herold <gherold(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-Hide quoted text - >> >> >> - 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
From: John Larkin on 26 Jul 2010 13:25 On Sat, 24 Jul 2010 21:14:13 -0700 (PDT), George Herold <gherold(a)teachspin.com> wrote: >On Jul 24, 11:51�am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Sat, 24 Jul 2010 09:45:53 +0100, Martin Brown >> >> >> >> >> >> <|||newspam...(a)nezumi.demon.co.uk> wrote: >> >On 23/07/2010 23:52, Jim Thompson wrote: >> >> Let's Take A Vote... >> >> >> While I write this up, hopefully sometime this weekend, let me ask for >> >> votes... >> >> >> How many think, as Larkin opines, "charge is not conserved" ?? >> >> >Hopefully not too many. But it is difficult to predict the behaviour of >> >electronics engineers - about half of them think Einstein was wrong :( >> >> >> How many think charge IS conserved ?? >> >> >Just about every physicist on the planet since Ben Franklin. >> >> >It was the inconsistency of Ampere's Law with conservation of charge >> >that led Maxwell to formulate his famous equations and show that >> >oscillating fields of electromagnetic radiation travel at a constant >> >speed c in a vacuum. >> >> >> Just curious what I'm up against here. >> >> >> � � � � � � � � � � � � � � � � � � � � �...Jim Thompson >> >> >A idealised physics version of your original capacitor problem but >> >without the switch can be stated as the following problem: >> >> >Two identical metal spheres with capacitance C are used. >> >Initially one is uncharged and the other with a charge Q >> >> >They are brought together from infinity until they touch. >> >> >Describe what happens and how the charge is distributed after they are >> >in electrical contact. You can add an infinite ground plane under the >> >experiment if it makes you feel better about the circuit analogue. >> >> Where can I buy 0.33 uF surface-mount metal spheres? Are they >> expensive? I'd need ROHS, of course, on reels. [1] >> >> John >> > >"[1] extra credit: how big would they be?" > > >epsilon sub zero times R? or is there a 4*pi in there? 4*pi*epsilon >is about 10^-10, so you need a radius of 10^3 meters for 0.1uF, 3.3 >kilometers for your desired value.... but I'm going to have to check >my math before I can quote you a price. > >George H. >- Hide quoted text - >> >> - Show quoted text - C goes linearly with radius, and it's conveniently close to 1 pf per cm. John
From: George Herold on 26 Jul 2010 14:25 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-Hidequoted text - > > >> >> - 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. 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) George H.
From: John Larkin on 26 Jul 2010 14:55
On Mon, 26 Jul 2010 11:25:26 -0700 (PDT), George Herold <gherold(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-Hidequoted text - >> >> >> >> - 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. Another nice program is Txline 2003, which does some cases that Appcad doesn't. I have a bunch of Terman's books, but I am *not* an old fart! Terman apparently was... read "The Inventor and the Pilot" for some dirt. John |