On Wed, 14 Sep 2016 08:52:02 +1000, Clifford Heath <no.spam@please.net> wrote:>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.Fig 3 in that doc is the sort of txline model I mentioned, an LT Spice ideal txline, with infinite common-mode rejection, and an added inductor to simulate the common-mode impedance of a coax shield. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Coax modelling question
Started by ●September 13, 2016
Reply by ●September 13, 20162016-09-13
Reply by ●September 13, 20162016-09-13
On Tuesday, September 13, 2016 at 4:07:30 PM UTC-4, John Larkin wrote:> 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. >OK I've only done it with a big spool (~100') of coax. The great thing about hitting it with a nice sharp edge is you get all the frequencies at one shot. (The hard part, (I'm guessing) is figuring out what it means.) But looking for difference's is easy enough. 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 18:01:06 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Tuesday, September 13, 2016 at 4:07:30 PM UTC-4, John Larkin wrote: >> 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. >> >OK I've only done it with a big spool (~100') of coax. >The great thing about hitting it with a nice >sharp edge is you get all the frequencies at one shot. >(The hard part, (I'm guessing) is figuring out what it means.) >But looking for difference's is easy enough. > >George H.TDR is approximately a graph of impedance vs distance. The vertical scale is actually reflection coefficient, from 0 ohms to infinite ohms over the span of 0 volts to Vgen. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●September 14, 20162016-09-14
On Tuesday, September 13, 2016 at 10:27:32 PM UTC-4, John Larkin wrote:> On Tue, 13 Sep 2016 18:01:06 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > > >On Tuesday, September 13, 2016 at 4:07:30 PM UTC-4, John Larkin wrote: > >> 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. > >> > >OK I've only done it with a big spool (~100') of coax. > >The great thing about hitting it with a nice > >sharp edge is you get all the frequencies at one shot. > >(The hard part, (I'm guessing) is figuring out what it means.) > >But looking for difference's is easy enough. > > > >George H. > > TDR is approximately a graph of impedance vs distance. The vertical > scale is actually reflection coefficient, from 0 ohms to infinite ohms > over the span of 0 volts to Vgen.Yeah I get it "in theory", I've never done it "in practice". It would be a nice technique to put in my bag of tricks. Is it typically done with steps or pulses? https://en.wikipedia.org/wiki/Time-domain_reflectometer Silly question, Doesn't zero ohms, give -Vgen, 50 ohms = 0 and open = +Vgen? George H.> > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
Reply by ●September 14, 20162016-09-14
>Is it typically done with steps or pulses? >https://en.wikipedia.org/wiki/Time-domain_reflectometer>Silly question, Doesn't zero ohms, give -Vgen, 50 ohms = 0 and open = +Vgen?Yes, but sitting on top of the step, so it's 0, +Vgen, and +2Vgen. Cheers Phil Hobbs
Reply by ●September 14, 20162016-09-14
On Wednesday, September 14, 2016 at 10:33:31 AM UTC-4, Phil Hobbs wrote:> >Is it typically done with steps or pulses? > >https://en.wikipedia.org/wiki/Time-domain_reflectometer > > >Silly question, Doesn't zero ohms, give -Vgen, 50 ohms = 0 and open = +Vgen? > > Yes, but sitting on top of the step, so it's 0, +Vgen, and +2Vgen. > > Cheers > > Phil HobbsGot it, a step then... I should make one. (someday.) https://www.youtube.com/watch?v=9cP6w2odGUc Does the 74AC series have the fastest edges? (Without going ECL crazy.) George H.
Reply by ●September 14, 20162016-09-14
On Wed, 14 Sep 2016 07:10:42 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Tuesday, September 13, 2016 at 10:27:32 PM UTC-4, John Larkin wrote: >> On Tue, 13 Sep 2016 18:01:06 -0700 (PDT), George Herold >> <gherold@teachspin.com> wrote: >> >> >On Tuesday, September 13, 2016 at 4:07:30 PM UTC-4, John Larkin wrote: >> >> 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. >> >> >> >OK I've only done it with a big spool (~100') of coax. >> >The great thing about hitting it with a nice >> >sharp edge is you get all the frequencies at one shot. >> >(The hard part, (I'm guessing) is figuring out what it means.) >> >But looking for difference's is easy enough. >> > >> >George H. >> >> TDR is approximately a graph of impedance vs distance. The vertical >> scale is actually reflection coefficient, from 0 ohms to infinite ohms >> over the span of 0 volts to Vgen. > >Yeah I get it "in theory", I've never done it "in practice". It would be >a nice technique to put in my bag of tricks. >Is it typically done with steps or pulses? >https://en.wikipedia.org/wiki/Time-domain_reflectometerElectrical is usually steps, optical is usually a narrow pulse.> >Silly question, Doesn't zero ohms, give -Vgen, 50 ohms = 0 and open = +Vgen? >Given a 50 ohm step generator, +1 volt unloaded, and the hi-z scope at the generator end, shorting the rig makes zero volts, and open circuit makes +1 volt. A 50 ohm coax (or resistor) loads the gen to 0.5 volts. I have an ancient Tek 11802 with the SD24 TDR head, about $1000 or so total on ebay. It makes a very clean ~~30 ps TDR step, so it can measure the impedance of short cables or PCB traces. It's dual channel, so it can also terminate and measure the signal from the far end of the DUT, the "TDT" signal. Like this: upper trace is TDR, lower is TDT. https://dl.dropboxusercontent.com/u/53724080/TDR/Z392_Meander.JPG https://dl.dropboxusercontent.com/u/53724080/TDR/Z392_TDR.jpg There is some HF loss in that trace, which trashes the risetime. https://dl.dropboxusercontent.com/u/53724080/TDR/Z392_Rise.JPG This is a fundamental tool for any sort of time-domain high frequency design. https://dl.dropboxusercontent.com/u/53724080/TDR/V880tdr.gif -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●September 14, 20162016-09-14
On Wed, 14 Sep 2016 08:46:25 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Wednesday, September 14, 2016 at 10:33:31 AM UTC-4, Phil Hobbs wrote: >> >Is it typically done with steps or pulses? � >> >https://en.wikipedia.org/wiki/Time-domain_reflectometer >> >> >Silly question, Doesn't zero ohms, give -Vgen, 50 ohms = 0 and open = +Vgen? >> >> Yes, but sitting on top of the step, so it's 0, +Vgen, and +2Vgen. >> >> Cheers >> >> Phil Hobbs > >Got it, a step then... I should make one. (someday.) >https://www.youtube.com/watch?v=9cP6w2odGUc > >Does the 74AC series have the fastest edges? (Without going ECL crazy.) > >George H.AC is usually sub-ns. Some of the Tiny Logic parts get to around 600 ps. Parallel all three sections of a NL37WZ16 for a brutal step generator. But ECL crazy is better, if you have a fast scope. I have a pcb layout for a fast, cheap, maybe 60 ps TDR, but I haven't had time to build it. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●September 14, 20162016-09-14
On 09/14/2016 11:59 AM, John Larkin wrote:> On Wed, 14 Sep 2016 07:10:42 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > >> On Tuesday, September 13, 2016 at 10:27:32 PM UTC-4, John Larkin wrote: >>> On Tue, 13 Sep 2016 18:01:06 -0700 (PDT), George Herold >>> <gherold@teachspin.com> wrote: >>> >>>> On Tuesday, September 13, 2016 at 4:07:30 PM UTC-4, John Larkin wrote: >>>>> 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. >>>>> >>>> OK I've only done it with a big spool (~100') of coax. >>>> The great thing about hitting it with a nice >>>> sharp edge is you get all the frequencies at one shot. >>>> (The hard part, (I'm guessing) is figuring out what it means.) >>>> But looking for difference's is easy enough. >>>> >>>> George H. >>> >>> TDR is approximately a graph of impedance vs distance. The vertical >>> scale is actually reflection coefficient, from 0 ohms to infinite ohms >>> over the span of 0 volts to Vgen. >> >> Yeah I get it "in theory", I've never done it "in practice". It would be >> a nice technique to put in my bag of tricks. >> Is it typically done with steps or pulses? >> https://en.wikipedia.org/wiki/Time-domain_reflectometer > > Electrical is usually steps, optical is usually a narrow pulse.I recently got a chance to try out an OTDR, for an expert witness case. They're fun. The main difference from electrical is that you get a pretty strong Rayleigh backscatter signal that allows a direct measurement of the fibre loss. (Also just put in an offer on a _brand new_ 2012 Mustang convertible with a 6-speed manual. They're even more fun.) 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 14, 20162016-09-14
On 14/09/16 15:10, George Herold wrote:> Is it [TDR] typically done with steps or pulses?Electrically it is done with either. For example... The 1975 vintage Tek 1502 uses a step ~200mV step in a 50ohm line; the system's reflected edge risetime is 140ps, 0.6" resolution. The similar vintage Tek 1503 uses a 10/100/1000ns half-sine pulse, 5V in 50ohm line. Much longer range than the 1502, unsurprisingly. CRTs, battery operated, operable with cover off while being splashed with water, non-operating can be submerged to 3ft. CRT mechanically isolated from chassis, so withstands 12" drops onto concrete. No wonder the military liked them.
