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Re: gEDA-user: Spice Transmission Line Simulation Question

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.

I get funny results. I use 

ac dec 100 1 1000meg
plot db(v(4)/v(2))

I see what I expect up to about 18mHz and then it oscillates.

What am I doing wrong? 

a couple of things.  See below.

tomdean

RG-8/U Simulation
* Z    L        C       V    TD     Rc           Rs            vswr    f
*      (uh/ft) (uf/ft) (%c) (ns/ft) (ohm/1000ft) (ohm/1000ft)          (mhz)
*50    .065    24.8    82   1.24    1.2          1.1           1.43:1  2250
*51.5  .059    22.5    85
* 10 ft line
.model inner LTRA (L=.065uh C=24.8pf R=.0012 len=10)
.model outer LTRA (L=.065uh C=24.8pf R=.0011 len=10)
.subckt rg8u_10 a1 a2 a3 a4
o1 a1  a2  a3  a4 inner
*o2 a2  0   a4  0  outer
.ends
*v1 1 0 0 c pulse 0 1 1ns 0.1ns
v1 1 0 dc 0 ac 1
r1 1 2 50
r2 4 0 50
X1 2 0 4 0 rg8u_10
* ac dec 100 1 1000meg
* plot db(v(4)/v(2))
.end


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.

The other is your expectation of what you should get. You have put in a transmission line with z0 = 51.2 Ohms and you're terminating both ends with 50 ohms. So you'll see variation in v(2) with frequency which is periodic with the line going through integer half wavelengths. In your example, the velocity is 7.876e8 m/s (yes, by confusing meters and feet in the input units you have a line faster than is possible). 10 meters is 1/2 a wavelength at 39.4 MHz with the numbers in your model. You'll note that your response is periodic with a period of 39.4 MHz.

So in fact the simulation shows exactly what you should see. The 0.2 dB of ripple should make sense too. It is pretty easy calculation just based on reflection coefficients. Hint: ignore the loss, find input reflection coefficient and then make use of the assumption of no loss and you can find the voltage ratio.

-Dan




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