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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