15
Library Models And Modeling
This section provides an overview of available models and presents modeling techniques and
considerations.
Library Models
OrCAD's libraries (PSpice A/D and PSpice) contain lossy transmission lines for coax and twisted pair
(frequency domain analysis only), as well as other distributed and lumped macromodels.
Coax Modeling
Modeling L and C
A simple for formula for the characteristic impedance of coax is
Where d
1
is the diameter of the inner conductor, and d
2
is the diameter of the inside surface of the shield.
The propagation delay is
Modeling R and G
For coaxial lines, the primary loss is from the skin effect. The resistance per unit length becomes:
Where 0 < m < 1. For coax inductance and capacitance per unit length can be treated as frequency
independent. Conductance per unit length is
Where C is capacitance per unit length, is the angular frequency, and tan is a dielectric material
coefficient ("loss tangent"). The angle is called the dielectric loss angle. This angle is usually quite small
(<0.005 radians) for most dielectrics up to RF frequencies.
The following is an example model of RG6A/U coax from OrCAD's libraries. Note that R and G use the
Laplace variable 's' to model attenuation as a function of frequency:
) ln(
60
1
2
0
d
d
Z
r
) / ( 85 in ps t r d
m
dc
Ks R R
tan C G
