Re: gEDA-user: Spice Transmission Line Simulation Question

[John Doty <jpd@xxxxxxxxxxxxx>]

On Aug 7, 2006, at 4:39 PM, Dan McMahill wrote:

> al davis wrote:
> > On Monday 07 August 2006 10:01, Dan McMahill wrote:
> > > the manual I looked at said that you specify L/C in H/m and
> > > F/m for the ltra model.  You seem to have specified it in
> > > terms of per foot.  So this is one problem.
> > That doesn't matter if you are consistent.  What matters is total  
> > L, total C, etc.  The ability to specify per unit length, and  
> > length is a convenience.
>
> It didn't look to me like this was an option.  As near as I can  
> tell from the model, you specify incremental values and so the  
> denominator does matter or you get the wrong velocity on the line.   
> In fact a quick sim of his netlist confirms that the velocity is  
> way off.

Al's right. If you use (henrys/foot, farads/foot) you get the correct  
velocity in feet/s. If you then use feet for the length, you get the  
correct electrical length in seconds, which is what SPICE really  
cares about. Note that the length units cancel in the Z0 calculation,  
so again being consistent is what matters.

John Doty              Noqsi Aerospace, Ltd.
jpd@xxxxxxxxxxxxx




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