Let's Take A Vote...
in [Design]
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From: Martin Brown on 28 Jul 2010 03:23 On 27/07/2010 20:06, Phil Hobbs 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 >>>>> 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. There speaks someone who by his own admission relies on futzing with the numbers instead of algebra or understanding the 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. I always loved the analytical symmetry tricks and conformal transformations. But in my era computers were the method of choice. How come you have read Morse&Fishback Methods of Theoretical Physics in two massive volumes and never found the time to flick through Feynman? (the latter is a lightweight by comparison - you could flash read it) > > (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.) The nasty bit is allowing for frequency dependent surface current skin effects in real conductors and computing accurately the fringe fields on awkwardly shaped solenoids. A trick managed in practice by deviously shaped pole pieces on the top flight mass spectrometers. There is a strong patent by one maker that still prevents other manufacturers from having a perpendicular focal plane for their collectors. I worked on porting ion optics programs from mainframes very early on. Regards, Martin Brown
From: Robert Baer on 28 Jul 2010 03:52 John Fields wrote: > On Tue, 27 Jul 2010 07:01:58 -0700, John Larkin > <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Tue, 27 Jul 2010 04:00:22 -0500, John Fields >> <jfields(a)austininstruments.com> wrote: >> >> >>>> It has to do with >>>> getting SI units right. Did you ever read the wiki piece on >>>> dimensional analysis? Do you think it is smoke and mirrors? >>>> >>>> So, where did I say that charges can't generate forces? If you can't >>>> find such a statement, YOU are the one with emotions clouding your >>>> reason. >>> --- >>> Nonsense. >>> >>> All it means is that its location has slipped my mind, that the >>> message has been deleted or, who knows??? >> Who knows??? I know. You are deluded or just a liar. I would never say >> anything so silly. > > --- > You would, you have, and you will again, so you're the liar. > > "Latching relays have infinite gain." is a pretty silly thing to say, > yes? > > > JF > I think i "made a case" that the "gain" was not too hot, using rough numbers for input power to switch states, and power handling capability. For an infinite "gain", either the power to switch states must be zero, and/or the power handling capability must be infinite. Clearly, NEITHER exists.
From: Robert Baer on 28 Jul 2010 03:57 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 > Makes sense that trimming trace width gives less and less an improvement as the (untrimmed) width decreases - reason? as you said, (capacitive) fringing.
From: John Larkin on 28 Jul 2010 10:03 On Wed, 28 Jul 2010 00:52:53 -0700, Robert Baer <robertbaer(a)localnet.com> wrote: >John Fields wrote: >> On Tue, 27 Jul 2010 07:01:58 -0700, John Larkin >> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>> On Tue, 27 Jul 2010 04:00:22 -0500, John Fields >>> <jfields(a)austininstruments.com> wrote: >>> >>> >>>>> It has to do with >>>>> getting SI units right. Did you ever read the wiki piece on >>>>> dimensional analysis? Do you think it is smoke and mirrors? >>>>> >>>>> So, where did I say that charges can't generate forces? If you can't >>>>> find such a statement, YOU are the one with emotions clouding your >>>>> reason. >>>> --- >>>> Nonsense. >>>> >>>> All it means is that its location has slipped my mind, that the >>>> message has been deleted or, who knows??? >>> Who knows??? I know. You are deluded or just a liar. I would never say >>> anything so silly. >> >> --- >> You would, you have, and you will again, so you're the liar. >> >> "Latching relays have infinite gain." is a pretty silly thing to say, >> yes? >> >> >> JF >> > I think i "made a case" that the "gain" was not too hot, using rough >numbers for input power to switch states, and power handling capability. > For an infinite "gain", either the power to switch states must be >zero, and/or the power handling capability must be infinite. > Clearly, NEITHER exists. Power gain is Pload/(Pcoil*DutyCycle), where Dutycycle is the fraction of time that the coil is energized. In plain English, power gain is averaged load power divided by averaged coil power. That has no upper bound as duty cycle approaches zero. In, say, a home thermostat that uses one AA battery, Dutycycle might be a few tens of PPM, which is why the battery will last a year or two. Probably the clock/LCD run the battery down more than the relay does. So the argument devolves to whether a number that is unboundedly large can be referred to as "infinite." Go for it. John
From: John Larkin on 28 Jul 2010 10:06
On Wed, 28 Jul 2010 08:23:40 +0100, Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote: >On 27/07/2010 20:06, Phil Hobbs 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 > >>>>>> 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. > >There speaks someone who by his own admission relies on futzing with the >numbers instead of algebra or understanding the physics. OK, show us the algebra for a general, closed-form solution for microstrip impedance. John |