18
r
r1
w1
= r 0.0000277821
g
g1
w1
= g 1.641254723810
14
The transmission line parameters derived are then
R={2.77821e-5*sqrt(s)}
G={-1.64125e-14*abs(s)}
Twinax and Shielded Twisted Pair (STP) Modeling
Twisted shielded pair (STP) or "twinax" is recommended for differential transmission systems at high
frequencies or in noisy environments. Although it has superior noise rejection, the proximity of the shield
increases the distributed capacitance which significantly attenuates the signal.
Modeling L, C, LM, and CM
To model STP for differential driving, a multiconductor transmission line is needed.
OrCAD library parts T2COUPLED or KCOUPLE2 may be used.
In addition to the lossy tline parameters R, L, G, C, you will need LM and CM, the mutual inductance
and capacitance between adjacent tlines per unit length
A simple for formula for the characteristic impedance of two parallel wires is
Where d is the diameter of the conductor, and s is the separation between wire centers. The propagation
delay is
For multiconductor (crosstalk) simulations, it is important to obtain the conductor to conductor coupling
parameters LM and CM. Here are suggestions for obtaining these parameters:
Contact cable vendor for mutual capacitance and inductance data.
Measure the capacitance and inductance matrices in the lab.
)
2
ln(
120
0
d
s
Z
r
) / ( 85 in ps t r d
