Laplace expression possible with LTSpice lossy transmission line?

Question

I'm trying to model a lossy transmission line in LTSpice. I want the simulation to work up to about 15GHz and include skin effect and frequency-dependant dielectric constant of pcb traces and material.

I know that in LTSpice it's possible to include Laplace expressions in signal generators. I was hoping that it would work in their lossy transmission, too, but there's nothing in their help files and nothing online.

When I use the simple model statement it works well (along with the appropriate .param statement for R, L, and C:

.model BobLossyTLine3inch LTRA(len=3 R={RperInch} L={LperInch} C={CperInch})

However, when I try to include some type of Laplace expression for resistance or capacitance, I can't find anything that will work. I suspect there's no way to do it.

Has anyone had any success with this?

Thanks.

Bob

cassiope · Accepted Answer

{CperInch})t=3DWhile we don't know much about LTSpice's internals, I suspect that thefractional exponents necessaryto model lossy transmission lines are beyond its capabilities.  i.e.there are no simple lumped-componentmodels, which I suspect would be necessary for any spice to use.I've done comparable things modeling fractional-exponent equivalentswith "enough" RLC sections tomake the approximation work.   You might try that.HTH...

oopere · Answer

Hi, BobIf I recall correctly, a transmission line may be modeled as (*):*---[Z]------            ------------------[Z]----*+       +   |+          +|           +            +V1      E1  O  F(V2,E1)  O F(V1,E2)  E2           V2-       -   |            |           -            -*------------            -------------------------*For an RLCG transmission line, you have s-dependent impedances Z while F has other s-dependent terms than exp(-s*T).You may simulate the impedances and the controlled source with the Laplace functions on almost any spice.(*) Google up: "method of characteristics" transmission linesPere

Laplace expression possible with LTSpice lossy transmission line?

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