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 PSpice: coax cable models 

Last post Tue, Jul 23 2013 5:56 AM by alokt. 3 replies.
Started by PavelM 21 Jul 2013 12:47 AM. Topic has 3 replies and 1447 views
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  • Sun, Jul 21 2013 12:47 AM

    • PavelM
    • Not Ranked
    • Joined on Mon, Jun 1 2009
    • Posts 2
    • Points 40
    PSpice: coax cable models Reply

    Hello,

    As far as I've checked, coax cable models in PSpice (TLine.lib library file, which could be open in any ASCII editor) have some definition which looks strange for me. At least, I don't understand where it came from. For example, for RG58C/U coax cable, for its both .model and .subckt options the model series resistance is defined as "r={63.9098u*sqrt(2*s)}". This resistance is intended to model the skin effect losses, which is proportional to sqrt(s).

    Later on in the model we can find the commented line with following "r={63.9098u*sqrt(PI2*frq)}". Here it is proportional to sqrt(2*pi*freq), which is a sqrt(s).

    Exactly the same file exists in the pre-ORCAD version - MSim.

    My question is why it is proportional to sqrt(2*s) in the active model and not to sqrt(s)? Where this factor 2 came from - mathematically / physically?

    Sincerely

    Pavel

    • Post Points: 20
  • Mon, Jul 22 2013 6:18 AM

    • alokt
    • Top 25 Contributor
    • Joined on Fri, Aug 22 2008
    • Noida, Uttar Pradesh
    • Posts 241
    • Points 3,470
    Re: PSpice: coax cable models Reply

    In tline library, each co-axial cable (RG58C/U) has two simulation model implementation

    One in form of .Model and other in form of .SUBCKT, which uses fixed frequency for model. depending upon how instance parameters in your schematic, one of these model implementation would be picked up.

    In both case R is function of SQRT(Frequency); i.e you should get similar attenuation/phase shift for both of these and both of these are active models.

    Hope this clarifies.

    • Post Points: 20
  • Mon, Jul 22 2013 7:32 AM

    • PavelM
    • Not Ranked
    • Joined on Mon, Jun 1 2009
    • Posts 2
    • Points 40
    Re: PSpice: coax cable models Reply

    Hi,

     You've explained why there are two types of model for each coax cable.

    My question is different.

    Below is a model of RG58C/U from the TLine.lib file:

    *******************************************************

    *             Z0(Ohms)  vp(%)  F1(MHz) Loss1(dB/100Ft) F2(MHz)  Loss2(dB/100Ft)
    * RG58C/U        50       66      100       4.9      1000        20
    .model RG58C/U  TRN (r={63.9098u*sqrt(2*s)} l=252.700n
    +   g={0.1584230p*abs(s)}   c=101.08p)
    *$
    * Subckt version uses fixed frequency, frq, to model simple lossy line
    *
    *                  Near end hi
    *                   |  Near end lo
    *                   |   |  Far end hi
    *                   |   |   |  Far end lo
    *                   |   |   |   |
    .subckt RG58C/U    A1  A2  B1  B2 params: frq=100Meg len=1
    .param PI2 {3.141592654*2}
    *.model RG58C/U  TRN (r={63.9098u*sqrt(PI2*frq)} l=252.700n
    *+   g={0.1584230p*PI2*frq}   c=101.08p)
    t A1 A2 B1 B2 rg58c/u len={len}
    .ends
    *$

    ***********************************************

    In line 3 of the model above, one can find following definition of R: r={63.9098u*sqrt(2*s)}

    Why it is proportional to (2*s) and not to (s), as it should be for "simple" skin effect? What is a physical explanation of it? Where this coefficient 2 came from?

    Thanks

    Sincerely

    Pavel

    • Post Points: 20
  • Tue, Jul 23 2013 5:56 AM

    • alokt
    • Top 25 Contributor
    • Joined on Fri, Aug 22 2008
    • Noida, Uttar Pradesh
    • Posts 241
    • Points 3,470
    Re: PSpice: coax cable models Reply

    I can not think of "physical" explanation for "2". R should be function of "SQRT(S)". To me physical interpretation still remain same even if I say "SQRT(2s)". R remains proportional to SQRT(s), however proportionality constant changes in both cases. Hope this clarifies.

    • Post Points: 5
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Started by PavelM at 21 Jul 2013 12:47 AM. Topic has 3 replies.