# How does one model tranmission line in SPICE simulation program

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#### matrixofdynamism

Models ususally emphasize certain properties of an object that matter while ignoring those that are not considered significant. Suppose one requires to carry out simulation of a high speed digital signal transmission and wants to determine how much reflection and ringing shall occur on the transmission line, how does one model the tranmission line? The idea here is to model it close to how the actual track is on the PCB. What approaches exist for this purpose and what are their pros and cons?

#### FvM

##### Super Moderator
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Did you already try with an ideal SPICE transmission line? It's a simple and straightforward way, supported by all SPICE variants. It has only two parameters Z and TD, which can be easily derived for PCB transmission lines.

The lossy transmission line model may be appropriate in some cases, but I don't see an advantage for the specified problem.

matrixofdynamism

### matrixofdynamism

Points: 2

#### matrixofdynamism

I do have model of an ideal transmission line and I have used it. But just out of curiosity, as far as I am aware, it should be possible to create a transmission line by using discrete passive circuit elements also. So how many capacitors, resistors and inductors would one need to make a transmission line with a given propagation delay?

#### tggzzz

Models ususally emphasize certain properties of an object that matter while ignoring those that are not considered significant. Suppose one requires to carry out simulation of a high speed digital signal transmission and wants to determine how much reflection and ringing shall occur on the transmission line, how does one model the tranmission line? The idea here is to model it close to how the actual track is on the PCB. What approaches exist for this purpose and what are their pros and cons?

For that objective, what benefit do you expect to get from lumped approximations?

If you want to model a via halfway along a line, simply use two half-length line models, plus whatever other components are necessary. If you want to model a stub, then add a stub-length line model.

matrixofdynamism

### matrixofdynamism

Points: 2

#### matrixofdynamism

The objective is to understand how its done, to know how things work "under the hood".

#### tggzzz

The objective is to understand how its done, to know how things work "under the hood".

In that case you should look at the source code for the relevant models. Spice is OSS.

I would be very surprised if the generated large numbers of lumped elements - that would be computationally inefficient.

#### FvM

##### Super Moderator
Staff member
Lumped LC delay lines have a cut-off frequency. You'll need 6 to 10 LC elements for 1 period delay at the maximum frequency.

matrixofdynamism

### matrixofdynamism

Points: 2
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