Re: gEDA-user: Spice Transmission Line Simulation Question

[al davis <ad106@xxxxxxxxxxxxxxxx>]

On Monday 07 August 2006 00:00, User Tomdean wrote:
> I am using Spice 3f5, attempting to model a coax line.  I am
> a nubie at this.  The lossless example uses two lines to
> model one coax.  Do I need to do this with the ltra model?
>  It does not seem to make a difference.

It has nothing to do with whether it is lossless or not.  The 
issue is that it only models one mode.   If you are using a 
coax, and really grounding the shield, and saying so by 
connecting to node 0, one is enough.

When you need two, the parameters will be very different.  The 
most common case is not for coax, but for a balanced line.  A 
balanced line has a differential mode and a common mode.  You 
can use single mode two transmission lines to model this.

The case where you would use two for a coax is where you want to 
model the desired transmission, using the center conductor and 
the shield as the line, and you also want to model a "signal" 
that might be carried on the shield, relative to real ground.  
Such propagation is usually undesirable.

If you need to ask this question, you probably do not have 
enough information to model this effect in a meaningful way, so 
you should just use a single transmission line.

If your subcircuit expansion results in all nodes of a device 
connected to 0, you can usually eliminate that device without 
consequence.


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