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OrCAD's transmission line library contains four UTP models:
Model Description Parameters
TP19AWG 19AWG UTP LENGTH, FREQ
TP22AWG 22AWG UTP LENGTH, FREQ
TP24AWG 24AWG UTP LENGTH, FREQ
TP26AWG 26AWG UTP LENGTH, FREQ
Table 6. UTP models in the transmission line library.
Note: These models can be used for transient analysis by setting FREQ=. This will
allow PSpice to use the R, G, and L values corresponding to . For use with AC sweep,
set FREQ=.
Modeling R and G at mid-range frequencies
Attenuation curve data is generally not available above ~16MHz for UTP cable. At these mid-range
frequencies, attenuation does not always obey a square root dependence on frequency. Here is a
suggested method to model UTP attenuation:
Obtain the frequency dependent R, G vs. frequency curves from the cable vendor.
Use a linear least square fitting routine to fit more points of the attenuation curve to a polynomial of
the form
Where s is complex frequency.
If only the attenuation data is available, follow the method used in [3], which is valid above ~500 KHz.
Assume that 90% of the loss is due to skin effect (the R parameter), and 10% due to dielectric loss
(the G parameter). Then, R=0.9*(2Z0)*attenuation, and G=0.1*(2/Z0)*attenuation. Note that this
model will considerably overestimate the loss at low frequencies.
Geometry Parameterized Models
Another way to model a transmission line is by describing its physical dimensions, and relative dielectric
constant. There are empirical equations derived for many popular transmission line geometries [6]. The
functions supported by PSpice's analog behavioral modeling expressions allow models to be created for
a large variety of geometries, including coaxial, paired, coplanar, microstrip, stripline, inverted microstrip,
and low order modes of waveguides. Coupled lines may also be parameterized by their geometry. Two of
the most common types are microstrip and stripline.
Microstrip Configuration
c
s b a
