On 09/13/2016 11:48 AM, John Larkin wrote:> On Tue, 13 Sep 2016 11:37:37 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 09/13/2016 11:31 AM, John Larkin wrote: >>> On Tue, 13 Sep 2016 10:16:13 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 09/13/2016 10:02 AM, Joseph Gwinn wrote: >>>>> On Sep 13, 2016, Syd Rumpo wrote >>>>> (in SED article <nr8eli$klu$1@dont-email.me>): >>>>> >>>>>> I can model a coax cable in LT spice using the inbuilt model or a lumped >>>>>> collection of R C and L. >>>>>> >>>>>> But the cable I want to model has no overall screen insulation, and >>>>>> somewhere between most and hardly any of it is very tightly wound onto a >>>>>> drum, so the outer on the drum is all 'shorted' together. The outer is >>>>>> steel and has the same order of resistance as the copper inner. >>>>>> >>>>>> That means there is part of the cable which looks like, I suppose, a >>>>>> mass of steel with an embedded coil. >>>>>> >>>>>> I don't have a cable to hand. >>>>>> >>>>>> How would you go about modelling this? I suppose that for the wound >>>>>> section, the R reduces, the C remains the same, but I'm not sure what to >>>>>> think about the L. >>>>> Transformer and lumped-element models fail here. The issue is that above a >>>>> critical frequency, outside and inside currents decouple due to skin effect. >>>>> In other words, above that frequency, shorting the outsides of the shields by >>>>> winding the coax on a metal rod has no effect on the currents on the inside >>>>> of the shield, even if you solder coax shield to rod. >>>>> >>>>> Joe Gwinn >>>>> >>>> >>>> The LTspice transmission line model fails even worse, because iirc it >>>> acts as a completely isolated, time-delayed source, as though it had a >>>> perfect balun attached. >>> >>> The LT Spice txline makes a nice 1:1 dc-daylight isolation >>> transformer. >>> >>> I don't think it qualifies as fails. If they didn't do it their way, >>> they would have to separately model the shield-to-universe impedance, >>> which would be hard to do. Or make it a 3-wire device. Or something. >>> As-is, you can add your own shield model, or just ground the shield >>> pins on both ends. >> >> I didn't mean that it failed altogether, just that I don't think it will >> work in the OP's application. I don't immediately see how you could use >> it to model a coax balun, even. >> > > It already models an ideal coax balun!One with infinite bandwidth, which is the problem.> > In the non-ideal case, one would connect something between the ground > pins of the two ends, to simulate the common-mode impedance. That > could be an inductor, or even another transmission line. And you could > add Z to ground on both ends, to sim coupling of the shield to the > universe. That is all far more complex than the simple Heaviside model > of a coax. > > I have broken a txline up into a lot of short segments, so that I can > add distributed-to-ground capacitance along the line. > > The LT Spice lines are fun, in that you can play games with the shield > currents. I could post a fun circuit, except that I'm still trying to > sell it.I haven't played with it much myself. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Coax modelling question
Started by ●September 13, 2016
Reply by ●September 13, 20162016-09-13
Reply by ●September 13, 20162016-09-13
On Tue, 13 Sep 2016 11:37:37 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 09/13/2016 11:31 AM, John Larkin wrote: >> On Tue, 13 Sep 2016 10:16:13 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 09/13/2016 10:02 AM, Joseph Gwinn wrote: >>>> On Sep 13, 2016, Syd Rumpo wrote >>>> (in SED article <nr8eli$klu$1@dont-email.me>): >>>> >>>>> I can model a coax cable in LT spice using the inbuilt model or a lumped >>>>> collection of R C and L. >>>>> >>>>> But the cable I want to model has no overall screen insulation, and >>>>> somewhere between most and hardly any of it is very tightly wound onto a >>>>> drum, so the outer on the drum is all 'shorted' together. The outer is >>>>> steel and has the same order of resistance as the copper inner. >>>>> >>>>> That means there is part of the cable which looks like, I suppose, a >>>>> mass of steel with an embedded coil. >>>>> >>>>> I don't have a cable to hand. >>>>> >>>>> How would you go about modelling this? I suppose that for the wound >>>>> section, the R reduces, the C remains the same, but I'm not sure what to >>>>> think about the L. >>>> Transformer and lumped-element models fail here. The issue is that above a >>>> critical frequency, outside and inside currents decouple due to skin effect. >>>> In other words, above that frequency, shorting the outsides of the shields by >>>> winding the coax on a metal rod has no effect on the currents on the inside >>>> of the shield, even if you solder coax shield to rod. >>>> >>>> Joe Gwinn >>>> >>> >>> The LTspice transmission line model fails even worse, because iirc it >>> acts as a completely isolated, time-delayed source, as though it had a >>> perfect balun attached. >> >> The LT Spice txline makes a nice 1:1 dc-daylight isolation >> transformer. >> >> I don't think it qualifies as fails. If they didn't do it their way, >> they would have to separately model the shield-to-universe impedance, >> which would be hard to do. Or make it a 3-wire device. Or something. >> As-is, you can add your own shield model, or just ground the shield >> pins on both ends. > >I didn't mean that it failed altogether, just that I don't think it will >work in the OP's application. I don't immediately see how you could use >it to model a coax balun, even. > >Cheers > >Phil HobbsLTspice (and PSpice) also includes a dissipative transmission line model. I don't know if even that is suitable for modeling transformers. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
Reply by ●September 13, 20162016-09-13
On Tue, 13 Sep 2016 12:15:51 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 09/13/2016 11:48 AM, John Larkin wrote: >> On Tue, 13 Sep 2016 11:37:37 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 09/13/2016 11:31 AM, John Larkin wrote: >>>> On Tue, 13 Sep 2016 10:16:13 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>>> On 09/13/2016 10:02 AM, Joseph Gwinn wrote: >>>>>> On Sep 13, 2016, Syd Rumpo wrote >>>>>> (in SED article <nr8eli$klu$1@dont-email.me>): >>>>>> >>>>>>> I can model a coax cable in LT spice using the inbuilt model or a lumped >>>>>>> collection of R C and L. >>>>>>> >>>>>>> But the cable I want to model has no overall screen insulation, and >>>>>>> somewhere between most and hardly any of it is very tightly wound onto a >>>>>>> drum, so the outer on the drum is all 'shorted' together. The outer is >>>>>>> steel and has the same order of resistance as the copper inner. >>>>>>> >>>>>>> That means there is part of the cable which looks like, I suppose, a >>>>>>> mass of steel with an embedded coil. >>>>>>> >>>>>>> I don't have a cable to hand. >>>>>>> >>>>>>> How would you go about modelling this? I suppose that for the wound >>>>>>> section, the R reduces, the C remains the same, but I'm not sure what to >>>>>>> think about the L. >>>>>> Transformer and lumped-element models fail here. The issue is that above a >>>>>> critical frequency, outside and inside currents decouple due to skin effect. >>>>>> In other words, above that frequency, shorting the outsides of the shields by >>>>>> winding the coax on a metal rod has no effect on the currents on the inside >>>>>> of the shield, even if you solder coax shield to rod. >>>>>> >>>>>> Joe Gwinn >>>>>> >>>>> >>>>> The LTspice transmission line model fails even worse, because iirc it >>>>> acts as a completely isolated, time-delayed source, as though it had a >>>>> perfect balun attached. >>>> >>>> The LT Spice txline makes a nice 1:1 dc-daylight isolation >>>> transformer. >>>> >>>> I don't think it qualifies as fails. If they didn't do it their way, >>>> they would have to separately model the shield-to-universe impedance, >>>> which would be hard to do. Or make it a 3-wire device. Or something. >>>> As-is, you can add your own shield model, or just ground the shield >>>> pins on both ends. >>> >>> I didn't mean that it failed altogether, just that I don't think it will >>> work in the OP's application. I don't immediately see how you could use >>> it to model a coax balun, even. >>> >> >> It already models an ideal coax balun! > >One with infinite bandwidth, which is the problem.Then add a lowpass filter in the signal path, or add some common-mode impedance. The ideal txline is just a starting point. Someone could create a better, paramaterized coax+balun model, based on the ideal line as a starting point, but it would be insanely complex. I just add the things that I figure will matter.> >> >> In the non-ideal case, one would connect something between the ground >> pins of the two ends, to simulate the common-mode impedance. That >> could be an inductor, or even another transmission line. And you could >> add Z to ground on both ends, to sim coupling of the shield to the >> universe. That is all far more complex than the simple Heaviside model >> of a coax. >> >> I have broken a txline up into a lot of short segments, so that I can >> add distributed-to-ground capacitance along the line. >> >> The LT Spice lines are fun, in that you can play games with the shield >> currents. I could post a fun circuit, except that I'm still trying to >> sell it. > >I haven't played with it much myself.People (not you, but more like beginners) can learn a lot by fiddling with the txline and pulse generators and resistors in LT Spice. Blumline sorts of things. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●September 13, 20162016-09-13
On 13/09/2016 16:34, John Larkin wrote:> On Tue, 13 Sep 2016 09:51:04 +0100, Syd Rumpo <usenet@nononono.co.uk> > wrote: > >> I can model a coax cable in LT spice using the inbuilt model or a lumped >> collection of R C and L. >> >> But the cable I want to model has no overall screen insulation, and >> somewhere between most and hardly any of it is very tightly wound onto a >> drum, so the outer on the drum is all 'shorted' together. The outer is >> steel and has the same order of resistance as the copper inner. >> >> That means there is part of the cable which looks like, I suppose, a >> mass of steel with an embedded coil. >> >> I don't have a cable to hand. >> >> How would you go about modelling this? I suppose that for the wound >> section, the R reduces, the C remains the same, but I'm not sure what to >> think about the L. >> >> Cheers > > Are you talking about a spool of insulated wire inside a metal bucket?I suppose a loosley wound spool of insulated wire in a cylinder of mercury might approximate.> That would have various amounts of wire-wire coupling, which will > cause all sorts of nasty effects. > > A sketch would help.Just take some coax, strip off the outer insulation and wind it on a drum.> > "somewhere between most and hardly any" makes modeling difficult.Yes, but that's the reality.> What is this for?A long cable (a wireline) for use in boreholes. The outer is steel armour for strength and has no insulation covering it. The inner is copper and there's a dielectric, probably PTFE. It's partly on and partly off a winch drum. Instruments at the bottom of the hole send signals to equipment at the top, sometimes vice-versa. I'm not sure what happens to the inductance of the part on the drum compared the the length in the borehole. You'll have the effect of the shorted outer, which must reduce overall inductance and resistance, and then the fact that the inner is making a big coil, with possibly many hundreds of turns on a 1 or 2 metre diameter. So does the inductance increase or decrease with winding? Cheers -- Syd
Reply by ●September 13, 20162016-09-13
On 09/13/2016 12:34 PM, Syd Rumpo wrote:> On 13/09/2016 16:34, John Larkin wrote: >> On Tue, 13 Sep 2016 09:51:04 +0100, Syd Rumpo <usenet@nononono.co.uk> >> wrote: >> >>> I can model a coax cable in LT spice using the inbuilt model or a lumped >>> collection of R C and L. >>> >>> But the cable I want to model has no overall screen insulation, and >>> somewhere between most and hardly any of it is very tightly wound onto a >>> drum, so the outer on the drum is all 'shorted' together. The outer is >>> steel and has the same order of resistance as the copper inner. >>> >>> That means there is part of the cable which looks like, I suppose, a >>> mass of steel with an embedded coil. >>> >>> I don't have a cable to hand. >>> >>> How would you go about modelling this? I suppose that for the wound >>> section, the R reduces, the C remains the same, but I'm not sure what to >>> think about the L. >>> >>> Cheers >> >> Are you talking about a spool of insulated wire inside a metal bucket? > > I suppose a loosley wound spool of insulated wire in a cylinder of > mercury might approximate. > >> That would have various amounts of wire-wire coupling, which will >> cause all sorts of nasty effects. >> >> A sketch would help. > > Just take some coax, strip off the outer insulation and wind it on a drum. >> >> "somewhere between most and hardly any" makes modeling difficult. > > Yes, but that's the reality. > >> What is this for? > > A long cable (a wireline) for use in boreholes. The outer is steel > armour for strength and has no insulation covering it. The inner is > copper and there's a dielectric, probably PTFE. It's partly on and > partly off a winch drum. Instruments at the bottom of the hole send > signals to equipment at the top, sometimes vice-versa. > > I'm not sure what happens to the inductance of the part on the drum > compared the the length in the borehole. You'll have the effect of the > shorted outer, which must reduce overall inductance and resistance, and > then the fact that the inner is making a big coil, with possibly many > hundreds of turns on a 1 or 2 metre diameter. > > So does the inductance increase or decrease with winding?Differential mode inductance will hardly change, since there's no B field outside the shield in that mode. Common mode inductance will change some. If the contact is poor, it'll go up on account of the mutual inductance of the turns, as usual. If the contact is good, it'll basically change from a long skinny wire to a short and very fat wire, so the inductance will go down. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●September 13, 20162016-09-13
On Tue, 13 Sep 2016 09:19:31 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:>On Tue, 13 Sep 2016 11:37:37 -0400, Phil Hobbs ><pcdhSpamMeSenseless@electrooptical.net> wrote: > >>On 09/13/2016 11:31 AM, John Larkin wrote: >>> On Tue, 13 Sep 2016 10:16:13 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 09/13/2016 10:02 AM, Joseph Gwinn wrote: >>>>> On Sep 13, 2016, Syd Rumpo wrote >>>>> (in SED article <nr8eli$klu$1@dont-email.me>): >>>>> >>>>>> I can model a coax cable in LT spice using the inbuilt model or a lumped >>>>>> collection of R C and L. >>>>>> >>>>>> But the cable I want to model has no overall screen insulation, and >>>>>> somewhere between most and hardly any of it is very tightly wound onto a >>>>>> drum, so the outer on the drum is all 'shorted' together. The outer is >>>>>> steel and has the same order of resistance as the copper inner. >>>>>> >>>>>> That means there is part of the cable which looks like, I suppose, a >>>>>> mass of steel with an embedded coil. >>>>>> >>>>>> I don't have a cable to hand. >>>>>> >>>>>> How would you go about modelling this? I suppose that for the wound >>>>>> section, the R reduces, the C remains the same, but I'm not sure what to >>>>>> think about the L. >>>>> Transformer and lumped-element models fail here. The issue is that above a >>>>> critical frequency, outside and inside currents decouple due to skin effect. >>>>> In other words, above that frequency, shorting the outsides of the shields by >>>>> winding the coax on a metal rod has no effect on the currents on the inside >>>>> of the shield, even if you solder coax shield to rod. >>>>> >>>>> Joe Gwinn >>>>> >>>> >>>> The LTspice transmission line model fails even worse, because iirc it >>>> acts as a completely isolated, time-delayed source, as though it had a >>>> perfect balun attached. >>> >>> The LT Spice txline makes a nice 1:1 dc-daylight isolation >>> transformer. >>> >>> I don't think it qualifies as fails. If they didn't do it their way, >>> they would have to separately model the shield-to-universe impedance, >>> which would be hard to do. Or make it a 3-wire device. Or something. >>> As-is, you can add your own shield model, or just ground the shield >>> pins on both ends. >> >>I didn't mean that it failed altogether, just that I don't think it will >>work in the OP's application. I don't immediately see how you could use >>it to model a coax balun, even. >> >>Cheers >> >>Phil Hobbs > >LTspice (and PSpice) also includes a dissipative transmission line >model. I don't know if even that is suitable for modeling >transformers. > > ...Jim Thompson<http://tinyurl.com/zuguclc> ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
Reply by ●September 13, 20162016-09-13
On Tue, 13 Sep 2016 17:34:56 +0100, Syd Rumpo <usenet@nononono.co.uk> wrote:>On 13/09/2016 16:34, John Larkin wrote: >> On Tue, 13 Sep 2016 09:51:04 +0100, Syd Rumpo <usenet@nononono.co.uk> >> wrote: >> >>> I can model a coax cable in LT spice using the inbuilt model or a lumped >>> collection of R C and L. >>> >>> But the cable I want to model has no overall screen insulation, and >>> somewhere between most and hardly any of it is very tightly wound onto a >>> drum, so the outer on the drum is all 'shorted' together. The outer is >>> steel and has the same order of resistance as the copper inner. >>> >>> That means there is part of the cable which looks like, I suppose, a >>> mass of steel with an embedded coil. >>> >>> I don't have a cable to hand. >>> >>> How would you go about modelling this? I suppose that for the wound >>> section, the R reduces, the C remains the same, but I'm not sure what to >>> think about the L. >>> >>> Cheers >> >> Are you talking about a spool of insulated wire inside a metal bucket? > >I suppose a loosley wound spool of insulated wire in a cylinder of >mercury might approximate. > >> That would have various amounts of wire-wire coupling, which will >> cause all sorts of nasty effects. >> >> A sketch would help. > >Just take some coax, strip off the outer insulation and wind it on a drum. >> >> "somewhere between most and hardly any" makes modeling difficult. > >Yes, but that's the reality. > >> What is this for? > >A long cable (a wireline) for use in boreholes. The outer is steel >armour for strength and has no insulation covering it. The inner is >copper and there's a dielectric, probably PTFE. It's partly on and >partly off a winch drum. Instruments at the bottom of the hole send >signals to equipment at the top, sometimes vice-versa. > >I'm not sure what happens to the inductance of the part on the drum >compared the the length in the borehole. You'll have the effect of the >shorted outer, which must reduce overall inductance and resistance, and >then the fact that the inner is making a big coil, with possibly many >hundreds of turns on a 1 or 2 metre diameter. > >So does the inductance increase or decrease with winding? > >CheersI'm guessing (free, no charge) that it will behave just like a straight coax. Winding it on the drum should have no effect beyond any mechanical distortion, and a little resistance change as the shield turns scrape. If the signal is balanced (no sneaky ground currents) the coax behaves like a N-ohm transmission line, where N can be calculated from the standard coax equation (or use Appcad.) There should be no additional inductance. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 13, 20162016-09-13
On Tuesday, September 13, 2016 at 3:34:35 PM UTC-4, John Larkin wrote:> On Tue, 13 Sep 2016 17:34:56 +0100, Syd Rumpo <usenet@nononono.co.uk> > wrote: > > >On 13/09/2016 16:34, John Larkin wrote: > >> On Tue, 13 Sep 2016 09:51:04 +0100, Syd Rumpo <usenet@nononono.co.uk> > >> wrote: > >> > >>> I can model a coax cable in LT spice using the inbuilt model or a lumped > >>> collection of R C and L. > >>> > >>> But the cable I want to model has no overall screen insulation, and > >>> somewhere between most and hardly any of it is very tightly wound onto a > >>> drum, so the outer on the drum is all 'shorted' together. The outer is > >>> steel and has the same order of resistance as the copper inner. > >>> > >>> That means there is part of the cable which looks like, I suppose, a > >>> mass of steel with an embedded coil. > >>> > >>> I don't have a cable to hand. > >>> > >>> How would you go about modelling this? I suppose that for the wound > >>> section, the R reduces, the C remains the same, but I'm not sure what to > >>> think about the L. > >>> > >>> Cheers > >> > >> Are you talking about a spool of insulated wire inside a metal bucket? > > > >I suppose a loosley wound spool of insulated wire in a cylinder of > >mercury might approximate. > > > >> That would have various amounts of wire-wire coupling, which will > >> cause all sorts of nasty effects. > >> > >> A sketch would help. > > > >Just take some coax, strip off the outer insulation and wind it on a drum. > >> > >> "somewhere between most and hardly any" makes modeling difficult. > > > >Yes, but that's the reality. > > > >> What is this for? > > > >A long cable (a wireline) for use in boreholes. The outer is steel > >armour for strength and has no insulation covering it. The inner is > >copper and there's a dielectric, probably PTFE. It's partly on and > >partly off a winch drum. Instruments at the bottom of the hole send > >signals to equipment at the top, sometimes vice-versa. > > > >I'm not sure what happens to the inductance of the part on the drum > >compared the the length in the borehole. You'll have the effect of the > >shorted outer, which must reduce overall inductance and resistance, and > >then the fact that the inner is making a big coil, with possibly many > >hundreds of turns on a 1 or 2 metre diameter. > > > >So does the inductance increase or decrease with winding? > > > >Cheers > > > I'm guessing (free, no charge) that it will behave just like a > straight coax. Winding it on the drum should have no effect beyond any > mechanical distortion, and a little resistance change as the shield > turns scrape. > > If the signal is balanced (no sneaky ground currents) the coax behaves > like a N-ohm transmission line, where N can be calculated from the > standard coax equation (or use Appcad.) There should be no additional > inductance. >This may not be practical for Syd, but I was thinking it would be nice to have data. Hit the big coax with a pulse or step, with the end open or shorted and see what the reflection looks like in both cases. (wound on spool and unwound.) George H.> > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●September 13, 20162016-09-13
On Tue, 13 Sep 2016 12:49:27 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Tuesday, September 13, 2016 at 3:34:35 PM UTC-4, John Larkin wrote: >> On Tue, 13 Sep 2016 17:34:56 +0100, Syd Rumpo <usenet@nononono.co.uk> >> wrote: >> >> >On 13/09/2016 16:34, John Larkin wrote: >> >> On Tue, 13 Sep 2016 09:51:04 +0100, Syd Rumpo <usenet@nononono.co.uk> >> >> wrote: >> >> >> >>> I can model a coax cable in LT spice using the inbuilt model or a lumped >> >>> collection of R C and L. >> >>> >> >>> But the cable I want to model has no overall screen insulation, and >> >>> somewhere between most and hardly any of it is very tightly wound onto a >> >>> drum, so the outer on the drum is all 'shorted' together. The outer is >> >>> steel and has the same order of resistance as the copper inner. >> >>> >> >>> That means there is part of the cable which looks like, I suppose, a >> >>> mass of steel with an embedded coil. >> >>> >> >>> I don't have a cable to hand. >> >>> >> >>> How would you go about modelling this? I suppose that for the wound >> >>> section, the R reduces, the C remains the same, but I'm not sure what to >> >>> think about the L. >> >>> >> >>> Cheers >> >> >> >> Are you talking about a spool of insulated wire inside a metal bucket? >> > >> >I suppose a loosley wound spool of insulated wire in a cylinder of >> >mercury might approximate. >> > >> >> That would have various amounts of wire-wire coupling, which will >> >> cause all sorts of nasty effects. >> >> >> >> A sketch would help. >> > >> >Just take some coax, strip off the outer insulation and wind it on a drum. >> >> >> >> "somewhere between most and hardly any" makes modeling difficult. >> > >> >Yes, but that's the reality. >> > >> >> What is this for? >> > >> >A long cable (a wireline) for use in boreholes. The outer is steel >> >armour for strength and has no insulation covering it. The inner is >> >copper and there's a dielectric, probably PTFE. It's partly on and >> >partly off a winch drum. Instruments at the bottom of the hole send >> >signals to equipment at the top, sometimes vice-versa. >> > >> >I'm not sure what happens to the inductance of the part on the drum >> >compared the the length in the borehole. You'll have the effect of the >> >shorted outer, which must reduce overall inductance and resistance, and >> >then the fact that the inner is making a big coil, with possibly many >> >hundreds of turns on a 1 or 2 metre diameter. >> > >> >So does the inductance increase or decrease with winding? >> > >> >Cheers >> >> >> I'm guessing (free, no charge) that it will behave just like a >> straight coax. Winding it on the drum should have no effect beyond any >> mechanical distortion, and a little resistance change as the shield >> turns scrape. >> >> If the signal is balanced (no sneaky ground currents) the coax behaves >> like a N-ohm transmission line, where N can be calculated from the >> standard coax equation (or use Appcad.) There should be no additional >> inductance. >> >This may not be practical for Syd, but I was thinking it would be nice to have data. >Hit the big coax with a pulse or step, with the end open or shorted and see what >the reflection looks like in both cases. (wound on spool and unwound.)That's TDR, which can be done with a pulse generator and a scope for longish runs. That could be mocked up pretty easily, with some stripped coax. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 13, 20162016-09-13
On 14/09/16 04:50, Jim Thompson wrote:> On Tue, 13 Sep 2016 09:19:31 -0700, Jim Thompson > <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: > >> On Tue, 13 Sep 2016 11:37:37 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 09/13/2016 11:31 AM, John Larkin wrote: >>>> On Tue, 13 Sep 2016 10:16:13 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>>> On 09/13/2016 10:02 AM, Joseph Gwinn wrote: >>>>>> On Sep 13, 2016, Syd Rumpo wrote >>>>>> (in SED article <nr8eli$klu$1@dont-email.me>): >>>>>> >>>>>>> I can model a coax cable in LT spice using the inbuilt model or a lumped >>>>>>> collection of R C and L. >>>>>>> >>>>>>> But the cable I want to model has no overall screen insulation, and >>>>>>> somewhere between most and hardly any of it is very tightly wound onto a >>>>>>> drum, so the outer on the drum is all 'shorted' together. The outer is >>>>>>> steel and has the same order of resistance as the copper inner. >>>>>>> >>>>>>> That means there is part of the cable which looks like, I suppose, a >>>>>>> mass of steel with an embedded coil. >>>>>>> >>>>>>> I don't have a cable to hand. >>>>>>> >>>>>>> How would you go about modelling this? I suppose that for the wound >>>>>>> section, the R reduces, the C remains the same, but I'm not sure what to >>>>>>> think about the L. >>>>>> Transformer and lumped-element models fail here. The issue is that above a >>>>>> critical frequency, outside and inside currents decouple due to skin effect. >>>>>> In other words, above that frequency, shorting the outsides of the shields by >>>>>> winding the coax on a metal rod has no effect on the currents on the inside >>>>>> of the shield, even if you solder coax shield to rod. >>>>>> >>>>>> Joe Gwinn >>>>>> >>>>> >>>>> The LTspice transmission line model fails even worse, because iirc it >>>>> acts as a completely isolated, time-delayed source, as though it had a >>>>> perfect balun attached. >>>> >>>> The LT Spice txline makes a nice 1:1 dc-daylight isolation >>>> transformer. >>>> >>>> I don't think it qualifies as fails. If they didn't do it their way, >>>> they would have to separately model the shield-to-universe impedance, >>>> which would be hard to do. Or make it a 3-wire device. Or something. >>>> As-is, you can add your own shield model, or just ground the shield >>>> pins on both ends. >>> >>> I didn't mean that it failed altogether, just that I don't think it will >>> work in the OP's application. I don't immediately see how you could use >>> it to model a coax balun, even. >>> >>> Cheers >>> >>> Phil Hobbs >> >> LTspice (and PSpice) also includes a dissipative transmission line >> model. I don't know if even that is suitable for modeling >> transformers. >> >> ...Jim Thompson > > <http://tinyurl.com/zuguclc>A timely pointer for me, thanks Jim. The TV transmitter modules that recently came into my possession use a very similar structure, though for 200MHz, and I was trying to figure out how the impedance transformers work. Clifford Heath.
