On Sun, 01 Mar 2015 03:39:23 -0700, RobertMacy <robert.a.macy@gmail.com> wrote:>On Sat, 28 Feb 2015 17:32:49 -0700, Allan Herriman ><allanherriman@hotmail.com> wrote: > >>> ....snip.... >> If you're just interested in Spice simulation (rather than actually >> building a lumped model out of physical components), perhaps this >> series of articles by old s.e.d. poster Roy McCammon might help: >> >> http://www.edn.com/electronics-blogs/anablog/4311804/Improved-Spice-model-of-a-transmission-line >> >> I haven't tried it, but it would seem to avoid some of the pitfalls >> of the usual lumped model. >> >> Regards, >> Allan > >Allan, > >Thanks for posting that URL. > >The article is circa 2011, so will post comment here instead: Roy mentions >non-noise sources of variations. But, NEVER mentions a true bane of all >cable manufacturers. Triboelectric effect. If the cable moves wind, >thermal flexing, whatever; the triboelectric effect will generate more >noise than one would think possible. The Manufacturers can reduce effect >depending on how 'tight' they can wrap that insulation around the >conductors and the material selection they use with teflon being VERY >energetic. Not sure, but expect to get worse with aging. > >Security Industry purposely use this effect to make 'sensor' cables. I >once took a foot long piece such cable on the bench, put a scope probe on >the center and shield, tapped the cable in the middle with the handle of a >screw driver to watch more than 8Vpp appear on the scope trace! Now THAT's >energetic!In FR4 PC boards, one can see (with a fast TDR) what is almost certainly the variation in dielectric constant caused by the fiberglas weave. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Ideal vs. lossy transmission line model question
Started by ●February 27, 2015
Reply by ●March 1, 20152015-03-01
Reply by ●March 1, 20152015-03-01
On Sun, 01 Mar 2015 09:20:05 -0800, John Larkin wrote:> On Sun, 01 Mar 2015 03:39:23 -0700, RobertMacy <robert.a.macy@gmail.com> > wrote:snip>> >>The article is circa 2011, so will post comment here instead: Roy >>mentions non-noise sources of variations. But, NEVER mentions a true >>bane of all cable manufacturers. Triboelectric effect. If the cable >>moves wind, thermal flexing, whatever; the triboelectric effect will >>generate more noise than one would think possible. The Manufacturers can >>reduce effect depending on how 'tight' they can wrap that insulation >>around the conductors and the material selection they use with teflon >>being VERY energetic. Not sure, but expect to get worse with aging. >> >>Security Industry purposely use this effect to make 'sensor' cables. I >>once took a foot long piece such cable on the bench, put a scope probe >>on the center and shield, tapped the cable in the middle with the handle >>of a screw driver to watch more than 8Vpp appear on the scope trace! Now >>THAT's energetic! > > In FR4 PC boards, one can see (with a fast TDR) what is almost certainly > the variation in dielectric constant caused by the fiberglas weave.Try G10.
Reply by ●March 1, 20152015-03-01
On Sun, 1 Mar 2015 17:21:54 +0000 (UTC), DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> wrote:>On Sun, 01 Mar 2015 09:20:05 -0800, John Larkin wrote: > >> On Sun, 01 Mar 2015 03:39:23 -0700, RobertMacy <robert.a.macy@gmail.com> >> wrote: >snip >>> >>>The article is circa 2011, so will post comment here instead: Roy >>>mentions non-noise sources of variations. But, NEVER mentions a true >>>bane of all cable manufacturers. Triboelectric effect. If the cable >>>moves wind, thermal flexing, whatever; the triboelectric effect will >>>generate more noise than one would think possible. The Manufacturers can >>>reduce effect depending on how 'tight' they can wrap that insulation >>>around the conductors and the material selection they use with teflon >>>being VERY energetic. Not sure, but expect to get worse with aging. >>> >>>Security Industry purposely use this effect to make 'sensor' cables. I >>>once took a foot long piece such cable on the bench, put a scope probe >>>on the center and shield, tapped the cable in the middle with the handle >>>of a screw driver to watch more than 8Vpp appear on the scope trace! Now >>>THAT's energetic! >> >> In FR4 PC boards, one can see (with a fast TDR) what is almost certainly >> the variation in dielectric constant caused by the fiberglas weave. > >Try G10.FR4 is just fire-retardant G10. Both are fiberglass/epoxy laminates. Neither is very well controlled as regards Er or very high frequency behavior. http://g10fr4.com/ -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●March 1, 20152015-03-01
On Sat, 28 Feb 2015 12:43:11 -0600, John S <Sophi.2@invalid.org> wrote:>IIRC, LTSpice has a lossy line model.O Lossy Transmission Line <http://ltwiki.org/index.php5?title=O_Lossy_Transmission_Line> T Lossless Transmission Line <http://ltwiki.org/index.php5?title=T_Lossless_Transmission_Line> O-device (Lossy Transmission Line) and T-device (Lossless Transmission Line) modeling issues <http://ltwiki.org/?title=O-device_%28Lossy_Transmission_Line%29_and_T-device_%28Lossless_Transmission_Line%29_modelling_issues> Modeling and Simulation of Nonlinear Transmission Lines by Frank Crowne, Army Research Laboratory <http://ltwiki.org/images/8/8d/Modeling_Transmission_Lines.pdf> -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Reply by ●March 2, 20152015-03-02
On Sun, 01 Mar 2015 10:20:05 -0700, John Larkin <jlarkin@highlandtechnology.com> wrote:>> .....snip.... > In FR4 PC boards, one can see (with a fast TDR) what is almost > certainly the variation in dielectric constant caused by the fiberglas > weave. > >didn't know that, but makes sense since the epoxy used has a different Er than glass fibre.
Reply by ●March 6, 20152015-03-06
On Saturday, February 28, 2015 at 7:33:12 PM UTC-5, Allan Herriman wrote:> On Fri, 27 Feb 2015 19:54:30 -0800, dakupoto wrote: > > > Could some electronics guru please clarify a few subtle questions > > regarding lumped parameter model of transmission lines ? > > > > The simple loss-less lumped parameter model consists of an ideal > > capacitor-inductor pair per unit length. > > > > OTOH, the lossy model includes a series resistance with the ideal > > inductor and an dielectric conductance in parallel with the ideal > > capacitor. > > > > Similarly, the non-ideal capacitor model includes a series resistance > > and a series inductance with the ideal capacitor. The non-ideal inductor > > model includes parasitic capacitance and resistance values. > > > > So, the question is: if one were to replace the ideal capacitor/indcutor > > pair in the lossless transmission line model with a non-ideal > > capacitor/inductor pair, then would this new model effectively re-create > > the lossy transmission line model ? I have done some SPICE modelling on > > this idea, > > and the results look encouraging. What do you gurus feel ? > > > > Any hints/suggestions would be greatly appreciated. Thanks in advance. > > If you're just interested in Spice simulation (rather than actually > building a lumped model out of physical components), perhaps this > series of articles by old s.e.d. poster Roy McCammon might help: > > http://www.edn.com/electronics-blogs/anablog/4311804/Improved-Spice-model-of-a-transmission-line > > I haven't tried it, but it would seem to avoid some of the pitfalls > of the usual lumped model. > > Regards, > AllanThanks for the reference. I had seen it a few years ago, but did not pay too close attention to it. This time, I examined it carefully, and some peculiar issues crop up. The basic scheme the author has followed is to use the frequency plane(s = jw) expressions for conductance, impedance, inductance, resistance and propagation constant and then take the inverse transform of these quantities in the SPICE code. This is perfectly fine in theory, because any complex expression may be split up in a partial fractions expansion, and then the inverse Laplace transform of each of these can parts may be obtained. The problem starts when one considers the s=plane expressions for impedance and propagation constant -- BOTH have square roots. And as far as I could see from my trusty copy of Abramovich and Stegun, BOTH forward and backward Laplace transforms for expressions with positive fractional exponents do not exist !!! For example, the s-plane expression for the frequency dependent resistance is: R(w) = Rdc(1 + (w/Wr)^2))^0.25 So how is the LTSpice engine going to evaluate the inverse transform for this expression ? In my humble opinion, a frequency plane solution with a simple C/C++ module with the input signal frequency being increased incrementally would provide a lot better solution -- I await each of your comments on this.
Reply by ●March 6, 20152015-03-06
On Fri, 06 Mar 2015 00:37:19 -0700, <dakupoto@gmail.com> wrote:>> ....snip.... > The basic scheme the author has followed is to > use the frequency plane(s = jw) expressions for > conductance, impedance, inductance, resistance > and propagation constant and then take the > inverse transform of these quantities in the > SPICE code. This is perfectly fine in theory, > because any complex expression may be split up > in a partial fractions expansion, and then the > inverse Laplace transform of each of these can > parts may be obtained. > The problem starts when one considers the s=plane > expressions for impedance and propagation > constant -- BOTH have square roots. And as far > as I could see from my trusty copy of Abramovich > and Stegun, BOTH forward and backward Laplace > transforms for expressions with positive > fractional exponents do not exist !!! > For example, the s-plane expression for the > frequency dependent resistance is: > R(w) = Rdc(1 + (w/Wr)^2))^0.25 > So how is the LTSpice engine going to evaluate > the inverse transform for this expression ? > In my humble opinion, a frequency plane solution > with a simple C/C++ module with the input signal > frequency being increased incrementally would provide a lot better > solution -- I await each of > your comments on this. >The last time I used a SPICE model with a 'frequency' term and tried to do a really useful analysis, like try to observe the expected change to the digital square wave, or obtain a useful set of 'eye patterns' for Error Rate Detection values; the analysis went from step, step, ..step.., ....step....., to predicting it might not finish in my lifetime, Stopping the analysis before anywhere nearly completed, it added insult to injury, the results had wild variations of error. so... I now make my OWN transmission line models using lumped models in small enough sections the error at maximum frequency [as caused by minimum risetime] PLUS, and this has been illuminating in understanding EMC emanations off a cable, it is possible to include 'free space' and actually estimate radiation from circuitry in a system that is not properly done. How to do Skin effect? I found that around 5 sets of elements, configured like eddy current models, inductor parallel with resistor feeding parallel inductor, etc can be made to pretty accurately 'curve fit' the resistance vs frequency, and a few more terms will even yield fairly accurate 'phase' shift from skin effects. [Note technique pretty accurately models those lossy RF Beads, somewhere I have a set of models for commercially available parts that are good to 1GHz, some beyond.] The advanatage of keeping the model frequency independent is that the model can be used for either .ac or .tran analyses. And, not take several days to RUN. Now, applied to transission lines, the model has conductor inductance and loss, return path inductance and loss [usually left out of lossy models], capacitance between conductor and return path, dielectric loss, AND the coupling [also left out of most models] which makes coax and twisted pair so desirable to use. At least with such a model you KNOW what's inside it. Also, you can really get to 'see' the dispersion in a cable. put in a step and watch the value step then 'slide' up to where it's supposed to go. The FREE PC Tools to create these models: femm 4.2; octave [Matlab clone]; and LTspice. [Of note, Mike Engelhardt, creator of LTspice, placed inside LTspice an 'array' function, which Alex Bordodynov has used to create incredible transmission line models. The array function makes it easy to have a very simple schematic containing a LOT of sections, 250 to more than 1000 sections with the schematic showing only a single little transmission line symbol. And, again since the model has NO frequency term it is easy to do either .ac or .tran analyses.
Reply by ●March 9, 20152015-03-09
On Friday, March 6, 2015 at 7:53:48 AM UTC-5, Robert Macy wrote:> On Fri, 06 Mar 2015 00:37:19 -0700, <dakupoto@gmail.com> wrote: > > >> ....snip.... > > The basic scheme the author has followed is to > > use the frequency plane(s = jw) expressions for > > conductance, impedance, inductance, resistance > > and propagation constant and then take the > > inverse transform of these quantities in the > > SPICE code. This is perfectly fine in theory, > > because any complex expression may be split up > > in a partial fractions expansion, and then the > > inverse Laplace transform of each of these can > > parts may be obtained. > > The problem starts when one considers the s=plane > > expressions for impedance and propagation > > constant -- BOTH have square roots. And as far > > as I could see from my trusty copy of Abramovich > > and Stegun, BOTH forward and backward Laplace > > transforms for expressions with positive > > fractional exponents do not exist !!! > > For example, the s-plane expression for the > > frequency dependent resistance is: > > R(w) = Rdc(1 + (w/Wr)^2))^0.25 > > So how is the LTSpice engine going to evaluate > > the inverse transform for this expression ? > > In my humble opinion, a frequency plane solution > > with a simple C/C++ module with the input signal > > frequency being increased incrementally would provide a lot better > > solution -- I await each of > > your comments on this. > > > > The last time I used a SPICE model with a 'frequency' term and tried to do > a really useful analysis, like try to observe the expected change to the > digital square wave, or obtain a useful set of 'eye patterns' for Error > Rate Detection values; the analysis went from step, step, ..step.., > ....step....., to predicting it might not finish in my lifetime, Stopping > the analysis before anywhere nearly completed, it added insult to injury, > the results had wild variations of error. so... > > I now make my OWN transmission line models using lumped models in small > enough sections the error at maximum frequency [as caused by minimum > risetime] PLUS, and this has been illuminating in understanding EMC > emanations off a cable, it is possible to include 'free space' and > actually estimate radiation from circuitry in a system that is not > properly done. > > How to do Skin effect? I found that around 5 sets of elements, configured > like eddy current models, inductor parallel with resistor feeding parallel > inductor, etc can be made to pretty accurately 'curve fit' the resistance > vs frequency, and a few more terms will even yield fairly accurate 'phase' > shift from skin effects. [Note technique pretty accurately models those > lossy RF Beads, somewhere I have a set of models for commercially > available parts that are good to 1GHz, some beyond.] The advanatage of > keeping the model frequency independent is that the model can be used for > either .ac or .tran analyses. And, not take several days to RUN. > > Now, applied to transission lines, the model has conductor inductance and > loss, return path inductance and loss [usually left out of lossy models], > capacitance between conductor and return path, dielectric loss, AND the > coupling [also left out of most models] which makes coax and twisted pair > so desirable to use. At least with such a model you KNOW what's inside it. > > Also, you can really get to 'see' the dispersion in a cable. put in a step > and watch the value step then 'slide' up to where it's supposed to go. > > The FREE PC Tools to create these models: femm 4.2; octave [Matlab clone]; > and LTspice. [Of note, Mike Engelhardt, creator of LTspice, placed inside > LTspice an 'array' function, which Alex Bordodynov has used to create > incredible transmission line models. The array function makes it easy to > have a very simple schematic containing a LOT of sections, 250 to more > than 1000 sections with the schematic showing only a single little > transmission line symbol. And, again since the model has NO frequency term > it is easy to do either .ac or .tran analyses.I am wholly in favor of the infinitesimal lumped element model, but have some questions about actual the values of these lumped capacitors/conductors/inductors. Specifically, so far I have found that the published values for resistance per unit length, capacitance per unit length etc., use the unit of length as either kilo-foot or kilometer. Resistance is directly proportional to length, so the resistance of e.g., a 0.5 centimeter unit length transmission line can be easily computed. But what about shunt conductance per unit length and more importantly inductance per unit length ? In addition, each of these parameters have frequency dependencies, but they can be tackled. Any hints/suggestions would be very helpful.
Reply by ●March 9, 20152015-03-09
On Sun, 08 Mar 2015 21:44:38 -0700, <dakupoto@gmail.com> wrote:>> ....snip... > I am wholly in favor of the infinitesimal > lumped element model, but have some questions > about actual the values of these lumped capacitors/conductors/inductors. > Specifically, so far I have found that the > published values for resistance per unit > length, capacitance per unit length etc., > use the unit of length as either kilo-foot > or kilometer. Resistance is directly > proportional to length, so the resistance > of e.g., a 0.5 centimeter unit length > transmission line can be easily computed. > But what about shunt conductance per unit > length and more importantly inductance per > unit length ? > In addition, each of these parameters have > frequency dependencies, but they can be > tackled. > Any hints/suggestions would be very helpful.Just as resistance per unit length gets divided down, so does 'reactance' per unit length. At a specific frequency, they are handled the same. Reactance just has a j term multiplied times it where j is sqrt(-1). +j is inductance and -j is capacitance, otherwise the reactance term is handled identically. Then to make sense of THAT, reactance per unit length, most people knowing the frequency, remove the frequency term, (actually radians term, 2pif,) and refer to reactance as either inductance or capacitance. Sadly, using terms like inductance and capacitance is misleading just because it implies NO change with frequency. and as you've seen even resistance is not the way to think, rather think in terms of 'loss' at a frequency, so ALL the cables' terms ultimately are a function of frequency. The better way to think is NOT R, L, and C but think in terms of Loss, Inductive Reactance, and Capacitive Reactance AT each specific frequency over the band of interest.
Reply by ●March 9, 20152015-03-09
On Monday, March 9, 2015 at 5:46:52 AM UTC-4, Robert Macy wrote:> On Sun, 08 Mar 2015 21:44:38 -0700, <dakupoto@gmail.com> wrote: > > >> ....snip... > > I am wholly in favor of the infinitesimal > > lumped element model, but have some questions > > about actual the values of these lumped capacitors/conductors/inductors. > > Specifically, so far I have found that the > > published values for resistance per unit > > length, capacitance per unit length etc., > > use the unit of length as either kilo-foot > > or kilometer. Resistance is directly > > proportional to length, so the resistance > > of e.g., a 0.5 centimeter unit length > > transmission line can be easily computed. > > But what about shunt conductance per unit > > length and more importantly inductance per > > unit length ? > > In addition, each of these parameters have > > frequency dependencies, but they can be > > tackled. > > Any hints/suggestions would be very helpful. > > Just as resistance per unit length gets divided down, so does 'reactance' > per unit length. At a specific frequency, they are handled the same. > Reactance just has a j term multiplied times it where j is sqrt(-1). +j is > inductance and -j is capacitance, otherwise the reactance term is handled > identically. Then to make sense of THAT, reactance per unit length, most > people knowing the frequency, remove the frequency term, (actually radians > term, 2pif,) and refer to reactance as either inductance or capacitance. > Sadly, using terms like inductance and capacitance is misleading just > because it implies NO change with frequency. and as you've seen even > resistance is not the way to think, rather think in terms of 'loss' at a > frequency, so ALL the cables' terms ultimately are a function of frequency. > > The better way to think is NOT R, L, and C but think in terms of Loss, > Inductive Reactance, and Capacitive Reactance AT each specific frequency > over the band of interest.I totally agree with you - I had been emphasizing the frequency dependency of each of the parameters all along. However, each of the capacitance, conductance, inductance and resistance per unit length, require a DC value as a basis/starting point for the analysis, and I had some doubts, which are now a lot clearer.
