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Adding transmission lines in spice simulation

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Advanced Member level 3
Nov 3, 2018
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If we know the capacitance per length and inductance per length as a model of microstrip and stripline, and also the length of the transmission line is also known then how can we add them in schematic and circuit simulation ? Would it be adding a inductor in series with inductance value for the total length of the transmission line on the PCB and a capacitor between signal line and the ground with capacitance in accordance to the length of the transmission line. For example, if the circuit in the schematic is involving a operational amplifier, which could be a pre-amplifier, a pulse shaper, a filter or could be a adc driver. If we add inductor and capacitor as transmission line parameters, and simulate in Spice, would that be simulating circuit with transmission line model ?

That will give you a very bent model of the tline, only one
"lump". More like a low pass LC filter. You are after the
opposite limit-case, arbitrarily-fine "lumps" that give you
a broadband flat-ish impedance.

Better to find a model which will run clean and realistic,
but I have yet to find one - so I just end up using ideal.

It really depends on the simulation purpose. I'm surprised that you ask about lumped LC transmission line model instead of referring to ideal SPICE transmission line. It's the obvious solution for most transmission line simulation problems in my view.

Special problems like transmission line with skin effect losses (cable model) can be best modeled by s-domain description, using laplace transform for time domain analysis. Lossy RLCG lumped transmission model achieve poor matching with real cables and are only good for small frequency range.

Falstad's animated interactive simulator supports modeling of transmission lines. Its Circuits menu includes several examples. (Screenshot below.)
Free to use and download:

transmission lines matched mismatched in Falstad's.png


I have used the lossy transmission line LTRA [1] in LTspice several times in combination with opamps up to the lower ~100 MHz range to model the interaction of electroinics and coaxial type transmission lines. The actual measurement results performed by a VNA, have been in good agreement with the simulation results iff parasitics of lumped components have been considered and the opamp model is not too primitive.

But as you are refering to striplines and microstrips, I assume you are interested in way higher frequencies.



Yes, I should ask for SPICE model of transmission lines. Does it exist ? Which model is used ? and can we add/include in the LT Spice library ?

Ideal transmission line (TLINE T) and lumped LRCG model (LTRA O) are basic SPICE circuit elements available in all derivates, also LTspice.

What are the parameters of these models that are available in LT Spice. Can we add trace length and width, dielectric thickness and the description of whole stackup. By doing this, can we simulate the signals in LT Spice if they are routed on microstrip and stripline transmission lines ? Can we also add vias and stubs ?

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