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Need a lumped lossy transmission line (L, R and G) that can be coupled (Lm)

NickW

Hi,

I'm looking for a PSpice transmission line part that includes resistance, inductance, conductance (between the conductors) and also mutual inductance between multiple transmission lines.

I can't use TLOSSY because its behavior is unstable under certain conditions that I need to simulate (e.g. 1 Meg Ohm resistor in series with one of the inputs simulating a break in the circuit). To my understanding, the distributed transmission line TLOSSY (and T) use an equation to relate the ports to each other (they are not actually electrically connected).

TLUMPx would be an ideal choice, except that the Kcouple2 won't couple TLUMPx's together (I think Kcouple2 works with the distributed transmission lines T and TLOSSY only).

 T2coupled, T3coupled, etc... look promising, but I can see only a single field for mutual inductance Lm, and I need different values of Lm for different combinations of pairs of lines. I also need to couple 6 lines to each other, whereas T5coupled (couples 5 lines) is the highest in the TLINE library.

I've been working on this for several days and need to come up with a solution soon. Any suggestions will be gratefully received!

Is there a PSpice part that will model L, R, G and Lm? Or will I have to create a new custom part?

Thank you,

Nick

  • Take a look in the tline.lib, (tools\pspice\library directory of the installation), you probably want to take a copy for your own use, or copy a section of interest to a text file as the basis for your own LIB file. The "common" Lm is a characteristic of the provided coupled parts ** and does appear to be a limitation of the implementation of the t<n>coupled parts. I don't know if it would be possible to create a custom part that implements differing LM between the lines, in any case, see the limitations in the tline.lib file and reference the Transmission line section of the PSpice Reference Manual to see what the capabilities of the inbuilt fundamental models are.

  • Thank you for your reply.

    I've had a look in tline.lib - the comments for 'tncoupled' say the parameters (including Lm) must be the same for all of the lines in the set. Also, coupling is modelled across adjacent lines only, whereas I need coupling between all lines. A modification to achieve this may be possible, but I'm not familiar with the format of library files (something I'll address as soon as I can).

    Unless the lumped models (TLUMP128 etc) can be coupled (I don't think they can) TLOSSY (in analog.lib) appears to be the only part that models all the features I require. I'll re-visit TLOSSY (and Kcouple2) today, and see if I can verify that I get sensible results under the input conditions in my model.

    Thanks again for your help :)

    Nick

  • Try a web search for Spice simulation of lumped transmission lines, I found a paper by Dhaene and De Zutter, "Selection of Lumped Element Models for Coupled Lossy Transmission lines", there were many other results, this paper discusses the simulatiuon issues encountered with such models and this paper, or another, may provide a usable solution to your modelling problem.

  • Since you need 6 coupled Tlines, you may require your own symbol. oldmouldy has already suggested a good way to start for that.
    Another alternative you may want to try is to create a subckt model (covering all tlines and couplings) and create a symbol for that. I feel that may be simpler approach for your need.
    Here is the syntax for 2 coupled lossy transmission line

    .subckt TL2 1 2 3 4
    T1 1 0 2 0 R=.31 L=.38u G=6.3u C=70p LEN=1
    T2 3 0 4 0 R=.29 L=.33u G=6.0u C=65p LEN=1
    K12 T1 T2 Lm=.04u Cm=6p

    .ends


    You can get the details about the format/syntax about Tline from PSpice A/D Reference Guide. You may have seen that already.

  • Thank you for your help oldmouldy and alokt.

    I found the paper "Selection of Lumped Element Models for Coupled Lossy Transmission lines". Lumped, coupled, lossy transmission lines is certainly what I am looking for.

    I'll also read up on how to create a subckt model.

    Thank you,

    Nick

  • As far as I can see, a subckt model comprises any combination of existing parts. I think I'm going to have to create a new part because my problem is that I cannot model what I need using the parts in the library as they stand (I need a lumped, lossy, coupled transmission line where for 6 lines, each pair of lines is coupled with a unique mutual inductance).

    Cheers,

    Nick

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